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Crankcase Breathing

A review of seemingly endless correspondence from NOC-L

This subject generated the longest running thread by far on NOC-L, running for months.

The postings discuss modifications using Positive Crankcase Ventilation valves (PCVs)

 Could an automotive PCV be used?  Linking the PCV into the inlet balance pipe
 Types and sizes of PCVs  An alternative to the Motormite PCV
 Results of Bob Patton's breather modification  Some simple solutions
 Experiences with the Motormite PCV  


"I have understood that the breather (located at the rear of left crankcase) is considered insufficient. What kind of problems does it cause, e.g. oil leaks caused by too high a pressure inside the engine?"


As you may be aware, the Norton motor acts like a giant air compressor during running. As the pistons go up and down the air within the case also expands and contracts. If the breather is not functioning properly oil will almost certainly leak out of the cracks and crevices. Most notably will be the timing chest as it seems to suffer the most. It could result in poor sealing of the main seals, and possible leaking around the primary chaincase where it attaches to the drive side case. If you have oil leaking out of the cases at any point (other than around the cylinder barrels and head) I would suspect breather problems, providing the motor is assembled in a proper fashion (Norton Service Release Notes October 1971).

Besides a multitude of other things, excessive oil consumption can be caused by an obstruction in the crankcase breather pipe. The factory notes that this can be checked by disconnecting the crankcase end of the pipe and blowing through (Note: I would recommend compressed air as oil is not very pleasing to the taste buds).

This is probably one of the most annoying parts for the Norton owner. Several changes were undertaken by the factory to keep the oil from spilling out onto the road (on a Norton?). For the 1968-1971 models, the breather was ported out of the drive side with a connection to the oil tank to prevent oil from flowing freely over the riders leg. The breather was timed off of the camshaft via a disc which was located on the end of the shaft and is very annoying to keep in place upon assembly of the cases. Generally speaking, this system worked well and has worked trouble free for me for 20-odd years.

The problems occur upon reassembly when the disc is improperly installed and either breaks or gets jammed. This causes either no breathing or it breathes all the time. Be extremely careful upon reassembly that this piece is properly fitted. The best way to tell is, upon reassembly push the cases together and they should slide to about 1/8" open. Then with mild pressure or light tapping with a rubber mallet try to get them together all the way. If they seem sluggish at this point then the disc has become misaligned and careful rotation of the camshaft is required to bring it all together.

In 1972 this setup was changed completely to a tube and separator on the aftmost position of the cases. This method was quickly abandoned in 1973 on the 850 only. The 850 had the best solution which was holes opened in the timing side case into the timing gear area and a breather tube affixed to the back side of the timing side where the blanking plate was situated. Care should be taken upon reassembly of this as the oil needs to flow back to the timing chest after running. Check for proper slope so there are no dead spots for oil to get trapped. This change can be used on the earlier models with good results. I have never bothered as my cases don't leak so the internal pressure must be fine. This particular setup was used throughout the 850 history. Upon reassembly check that the tube is clean and free of obstructions. Also make sure it is tight to all fittings as pressure can be quite high. Proper care of this component will keep the cases leak free and give better performance through proper breathing of the cases.

Steve Jowett (

Breather bottle to prevent oil emulsification

I had problems with the engine oil becoming emulsified on my Atlas during cold weather; well I think I've solved the problem. The timed breather on the end of the camshaft has now been blanked off, and the breather elbow is now fitted behind the timing cover, in between the points unit and the back of the crankcase. Instead of taking the breather pipe straight to the oil tank to recycle oil mist, it now goes up to a second pipe elbow on the inlet rocker cover, with a third breather pipe elbow fitted to one of the exhaust rocker covers. The vertical breather pipe from the crankcase to the head is very close to the barrel, so the gas stays hot and does not condense and emulsify on its way to the head. I connected the outlet breather pipe from the exhaust rocker cover to a breather bottle, and the only liquid that collected was a small volume of water. So this breather can now be vented direct to the atmosphere, and I won't have to fit any unsightly breather pipe unions to my new Manx replica central oil tank. (I just have unsightly breather pipe unions on the rocker covers instead). After a couple of hundred miles with this set up, there is no sign of emulsification of the oil in the tank at all, but with the standard breather arrangement it would have turned a nasty grey colour by now.

Andy Holmes ( on NOC-L 12th. Mar 1997

Too much oil finds its way out of the breather - could an automotive PCV be used?

There are of course three very different standard set ups of the rear crankcase breather on the Commando. The early style was an unchanged carryover of the old timed vent on the right end of the camshaft. Others presumably have the infamous Combat breather right off the crankcase, as do (did) I. This lasted very briefly in production, and if some find it satisfactory, very few others do, although when I just aimed my breather hose at the chain, I was at least insulated from the system's defects. Later I plumbed it into the air filter as original, and then I learned.

Brian Slark famously states that in factory tests of the arrangement, more oil was returned to the oil tank through this breather than the pumped return line. At first I was not sure why this is so bad, except that it does seem to contribute to my big complaint, which is that at engine speeds of 4000 r.p.m. or more, oil finds its way through the breather in such quantity that it ends up in the airbox, where the various turbulences and drafts cause much of it to blow out of the airbox (instead of being sucked into the carburettors as one might hope, for a little extra top end lubrication) and down the side of the bike as well as all over my pants. The standard way of dealing with this, as documented in all the club service notes, is to block off the crankcase breather, drill three holes from the right side of the crankcase into the timing chest, and plumb in a breather at the rear thereof; in other words, duplicate the design Norton used from 1973 until the end. I have done this and find some improvement, but certainly not all I would desire.

Discussions on Brit-Iron a few months ago lead me to believe that an automotive positive crankcase ventilation valve installed in the breather hose will improve the symptoms if not the underlying disease. More recently someone on Brit-Iron or NOC-L referred to a related design flaw in the Combat crankcase. He indicated that the oil pickup for the return pump was too far forward, and that by drilling new holes into an existing return oil passage at the rear of the case, the scavenging of oil from the crankcase would be greatly improved. He wrote as if it was a well known and documented modification, and when I posted that I had never seen or heard of it in 16 years of Norton ownership and would someone fill me in, there was no reply. It seems that this is something people would definitely want to do, if someone can provide specific directions.

Ben English ( on NOC-L 24th. Apr 1997

Use a small oil catch bottle in the oil tank breather line

I hope Ben is kidding us when he says he plumbed the breather into the air filter. I guess he means the oil tank breather, not the line from the back of the cases. But if his breather system is set up as it was when the bike was built, there can be no causal relationship between oil getting into the air filter from the tank vent and oil being pumped up the crankcase breather line by crankcase pressure. Just look at the system and think about it.

In my experience, the oil tank vent will put oil into the air filter only if the oil tank is overfilled (an easy thing to do because of the rapid drain-down into the cases with even a good-condition oil pump). If the oil tank is filled to the correct level right after the engine is stopped, then oil doesn't get blown through to the filter at any engine speed, even sustained >5,000 r.p.m. cruising.

The solution to the problem - and this is an oil tank breather problem, not an engine breather problem - is to blank off the hose stub on the air filter and route the tank breather overboard, preferably via a small catch tank (a 200 ml brake fluid bottle does for me). This set up has worked for me for ten years with no problems at all. If you don't overfill the oil tank you'll never see more than a few ccs of oil in there even after a 200-mile thrash. It's possible (but unlikely) that all the oil returns by the crankcase breather at high engine speeds, but as long as it gets back in the tank I'm not unduly concerned. Some folk will go on about aeration, etc, but my oil doesn't seem to do this. When I pop the tank cap after a long, fast burn there's just oil in there, not bubbles. When I took the crank out at 100,000 miles the (standard size) big end journals were judged good enough to go straight back in, were it not for a score mark caused by a bit of grit or metal (deteriorating camshaft probably!) that got in there. If you can do 100,000 hard miles on a crank with no regrind despite the 'limitations' of the crankcase breathing system, it's good enough for me. If Ben's bike (or anyone else's '72 model) is chucking engine oil out of the air filter when the breathing system is connected as per the original build, in my humble opinion they've simply filled the oil tank up too high.

Joe Schofield ( on NOC-L 25th. Apr 1997

Would a PCV pressurise the crankcase?    Is the oil mist impossible to separate out?

I was sloppy in my description, Joe is quite right. There is the engine breather, and then there is the oil tank breather. No matter how the engine breather is plumbed, one connects it to the oil tank in the same way. It is the breather from the oil tank that Norton plumbed into the airbox, and many simply dump on the chain as I did long ago when I had no airbox.

But wouldn't there some sort of relationship between how the motor is breathed, and what goes out the oil tank breather? For two reasons:

  • 1) Pressure variations from the crankcase. If I put a PCV valve in the breather, cancelling out the negative pressure waves from the engine, will the oil tank become pressurized, and the situation get worse?
  • 2) Oil returned by the scavenge line is pretty simply oil. What comes into the tank from the breather is a mist, in some sort of suspension. The air-oil mixture ratio may vary. Is a "rich" mixture resulting from a badly designed crankcase breather more susceptible to going out the tank breather, defeating all the separators and ending up in the airbox?

I am very careful to never let my oil level get much above the 'LOW' mark on the dipstick, and have learned to be happy even when it does not touch the dipstick. Still, it comes pumping out. But I should give the catch tank idea a whirl, just to see how much is really coming out that way. My rings are getting old and I may be burning more than I think.

Ben English ( on NOC-L 28th. Apr 1997

A PCV fitted in the oil tank breather works fine

I bought a new stainless steel air filter box for my 1974 Roadster from Colin Kelly. (He has a small business on Vancouver Island making very good stainless and mild steel sheet metal parts.) Anyway, it came without the welded-on tube that connects to the tank breather, as fitted to the 1974 models.

Instead, I plumbed the breather from the tank to the open, feeding the pipe behind the air filter and in front of the battery tray to just below the swing arm. I fitted a PCV non-return valve in this line, to prevent any dust being sucked back up, just in case. It works fine so far; there's usually just a hint of emulsified oil at the tip of the drain after a run. For the sake of a little lost oil, this avoids getting the paper filter element soaked in oil, which I'm told severely restricts its breathability.

Robert Smith ( on NOC-L 28th. Apr 1997

The catch bottle overflows on starting up when the machine has been standing

I put a bottle between the battery box and oil tank on my Atlas to catch the condensate from the crankcase breather (which otherwise contaminates the oil) and it works very well. What I did not think through is what happens when you don't use the bike for a while and the oil drains to the sump. When you start the bike, it pumps oil out the breather which fills the bottle in seconds then makes a big mess on the floor.

Peter Sprot ( on NOC-L 29th. Apr 1997

A breather tube and PCV from inlet valve cover to carburettor manifold

Today I put the fittings in for a breather tube from the intake valve cover to the manifold of my Mikuni VM-34. This was a bit tricky, as I had to drill a small hole for a 1/4-28 tap being careful not to drill all the way through.. I then turned a special fitting to fit this hole, the bottom which was drilled through with a .035 " drill for an orifice of just under 1mm. I then put in a Purolator PCV valve between the crankcase and oil tank so that the flow created an negative pressure in the crankcase. Rode it a bit, and no leaks thus far.

Steve Schoner ( on NOC-L 8th. Jul 1997

PCV from a Volvo not very satisfactory

On a related note, I have tried a PC valve from a pushrod Volvo of mid-seventies vintage in the breather system of my G80 Matchless. This is one of the few valve types that react to pressure rather than vacuum. You can hear it rattle at idle and slightly above, but much faster and it stops rattling. I think the inertia of the valve is too great for high frequency operation. What's really needed is an in-line reed-valve but I have yet to find one. Would anybody care to make a bunch for sale?

Thomas H. Allen ( on NOC-L 9th. Jul 1997

Matchless G80 valve might work on Nortons

I'm surprised Tom didn't mention the flap valve already installed in the G80 motors (the exit is behind the drive side bearing boss). It's a very light steel plate or diaphragm (9/16" diameter) which seats in/on a valve body. In my opinion it qualifies as a pressure valve, i.e., letting crankcase excess pressure out and preventing atmospheric pressure from entering. On average, it will render a partial vacuum in the crankcase. I have tested it in situ with new parts before writing this letter, so can vouch for its quality.

In my opinion this valve works so well - the diaphragm is very light so will probably work at high revs as well - that I am tempted to try it on the Norton twin. A new body replacing the tube bend normally fitted left of the camshaft boss is not too difficult to make. The only drawback I see is that it may be too small for the twin so I may have to scale it up a little.

Knut Soensteby ( on NOC-L 10th. Jul 1997

Crankcase aspiration volumes

For a single cylinder or a dual-cylinder engine with pistons operating in tandem, the crankcase is aspirated during each revolution according to the following table. The table is constructed with units familiar to those on both sides of the pond.

Displacement Cubic ft. / Minute Cubic ft. / Second Litres / Minute Litres / Second
400cc   71 1.2 2000 33.3
450cc   79 1.3 2250 37.5
500cc   88 1.5 2500 41.7
550cc   97 1.6 2750 45.8
600cc 106 1.8 3000 50.0
650cc 115 1.9 3250 54.2
700cc 123 2.0 3500 58.3
750cc 132 2.2 3750 62.5
800cc 141 2.4 4000 66.7
850cc 150 2.5 4250 70.8

The total displacement of the engine is vented out of the crankcase on each downstroke and is sucked in during each upstroke. Total air flow (both up- and downstrokes) is double the value in the table. i.e. an 850cc engine at 5000 r.p.m. must exhaust 150 c.f.m., and must ingest an additional 150 c.f.m., for a total air volume displacement of 300 c.f.m. (8500 litres per minute).

If the crankcase breather tube is restricted due to fouling, dents, or inadequate size, the aspiration rate will be reduced. If the required volume of air is not aspirated, the pistons will be fighting an opposing vacuum on the upstroke, and will be fighting a positive pressure on the downstroke, thus robbing your engine of power and reducing peak r.p.m. If you have piston blow-by due to ill fitting, worn, or fouled rings, the crankcase is overpressurized on the power stroke and you can throw the data-table right out of the window. A previous post on the air flow rates of PCV valves indicated a maximum flow rate of less than 5 c.f.m. As you can see from the chart above, this flow rate is grossly inadequate for even the smallest engine.

David L. Vann ( on NOC-L 10th. Jul 1997

Sizing PCVs

To achieve a complete pressure balance with ambient pressure you would have to cut a large (very large) hole in your crankcase. Both because of cleaning requirements and desired acceleration of oil draining from the top-end, it's not desirable to have a partial vacuum in the crankcase; at least on average of the crankshaft rotation cycle. Therefore the makers fit a pressure or vacuum valve ensuring pressure equivalence at minimum crankcase volume (at b.d.c. for our parallel twins), which gives a partial vacuum at t.d.c. and also on average crankcase volume (at mid-piston travel), provided the pressure valve closes entirely on the upward piston stroke, and there is no leak past the piston rings etc.

There will be no additional volume work from this: the work spent on the upward stroke causing vacuum is won on the downward stroke by suction. A PCV valve of 5 c.f.m. may therefore be adequate for the application. Most of its work is performed at low engine speeds until vacuum has been established. Thereafter, it works to adjust pressure balance at b.t.d. with rising temperature. My discussion assumes the engine is absolutely pressure tight. I am also making certain other assumptions of minor importance. In reality the engine is not pressure tight (crankshaft seals, valve guides, piston rings will all leak a certain amount) so the pressure valve will have to compensate for these leaks, which are rev and load dependent.

Knut Soensteby ( on NOC-L 11th. Jul 1997

Types and sizes of PCVs

I'm with Knut in thinking we do want a negative pressure in the crankcase of singles and 360o twins.

Yesterday I went back to my local self-serve chain auto parts store to fondle the PCV valves hanging in bubble packs. They carry only the Fram brand, and the oft mentioned Standard AV13 and AV23 were not on the crossover list. I decided to squander US$3.99 on a Fram FV307, as used on certain Chrysler products, I suspect ones with Mitsubishi motors. This valve is all metal. It has a 3/8" hose barb on one end, and the other is 1/8" pipe thread intended to screw into the engine block. This allowed me to figure out which end was which, a crucial piece of information not discernible on the other types. Once liberated from the packaging, I found that I could blow through it easily, although in the reverse direction, it does not stop all flow but instead places a notable restriction on it. The one way action seems to result from a small ball or something being blown one way or the other. I found a piece of brass pipe about 1/2" o.d. and cut 1/8" pipe threads inside it. After screwing this on the end intended for the Chrysler motor, it became a perfect replacement for the 1/2" to 3/8" reducer in the Commando breather line just as it nears the oil tank. I installed it last night, and will give it a test.

A previous post on the air flow rates of PCV valves indicated a maximum flow rate of less than 5 c.f.m. although it has also been said that this flow rate is 'grossly inadequate' for even the smallest engine. Actually, my lung power test makes me think that a lot more than five cubic feet cold be squeezed through the FV307's 1/4" hole in a minute. However, I do think it may prove inadequate for other reasons. I have a 6D917 air compressor check valve on order to try next, and Bob Patton and Steven R. Schoner's experiments in mind also.

Ben English ( on NOC-L 11th. Jul 1997

Matchless G80 valve details

Half a dozen owners have asked me for details of the Matchless model G3/G80 valve, fitted to all singles since early postwar models (at least). Not all of the parts can be ordered, due to part of the valve seat being in the crankcase - basically a boss and two concentric ring seats which the plate (diaphragm) closes against. The other parts are:-

  • 000835 Diaphragm, for release valve - 1 off
  • 000836 Body, release valve - 1 off

They should be easy to adopt to the Norton twin. I may eventually offer a dedicated housing (announcement to be made). Checking the native Matchless twin (500/600/650/750), it has a similar design, but breathing is through the crankshaft drive side axle with the breather fitted at the end of this. Incidentally, this is the very same design as the 7R/G50 motors have. The parts may therefore be even better suited for adoption. The parts include:-

  • 023260 Bolt, engine sprocket + crankcase release valve - 1 off
  • 023492 Diaphragm, for crankcase release valve - 1 off
  • 018282 Spring, for release valve diaphragm - 1 off
  • 016674 Plug, retaining release valve diaphragm (screws in bolt 023260) - 1 off

In later years (1960-66), the twin release valve was supplied in two different assembled versions, one for the alternator road models (500/650/750) and one for the dynamo equipped models 650DL/650CS/650CSR. The difference is related to the design of the crankshaft axle:-

  • 025298 Valve, release, assembled - 1 off - 500/650/750
  • 025526 Valve, release, assembled - 1 off - 650DL/650CS/CSR

The version most suitable for Nortons would be 025526. With different threads (unless you are very fortunate to find it fits), it would screw into the camshaft boss of the drive side crankcase (20M3 engine assumed) and you would have to make provision to adopt the breather pipe from there. On later engines - I would drop the 850cc style breather and retain the 20M3 or M15/M18 (600/650) design which is higher up on the crankcase. This may sound like a drastic measure but the emphasis must be on breathing and (air) pressure release, not oil pumping which is often what happens down there.

Getting away from the sump can only be beneficial with respect to the aim. By the way, spring 018282 lets you adjust opening pressure, a feature hardly found on an automotive PCV valve. Another part listed for the twin but not needed by most of you is:-

  • 023268 Washer, tab, engine sprocket bolt securing - 1 off

Sources for 025298 and 025526 include Russell Motors, London, U.K. @ GBP7 (around US$10.00) (net prices). Other sources include the AJSMOC (U.K.) and Accessory Mart in the U.S.A. (025298 only), also Walridge Motors, Canada.

Knut Soensteby ( on NOC-L 14th. Jul 1997

Other breather ideas

I've followed the thread on breathers and spent hours pondering the problem in my leisure moments. Four and eight cylinder engines don't have much of a problem as there is no net change in case volume as they turn over. In fact cars had road draft tubes to pull nasty fumes out until they were replaced by the omnipresent PCV system which does the same thing.

Looking at the size of most breather tubes it is obvious that the designers never intended to flow the entire displacement each revolution. With a reed (PCV) valve you will evacuate the case while kicking the engine over to start it. Theoretically then all we have to deal with is what blows past the rings and what leaks past the seals. In real life of course things are different.

Early Gold Stars had a reed valve breather with a very light fibre disc for the valve. As the factory developed the engine they ran into breathing troubles. They ran the engine on a dyno. with a manometer hooked to the crankcase. The maximum vacuum was at 2000 r.p.m., and then steadily decreased until it reached zero at 6000. They believed that this was due to the valve not being able to keep up with the cycles. They replaced it with a timed breather and never looked back. My friend C.R. Axtell did the same test with the later breather and varied the breather timing a bit, he also added a reed valve on the tappet adjuster plate. Ron Wood ran the timed breather on his Nortons with good results. Gene Romero's Triumphs all had timed breathers, although Ax did enlarge the case volume to drop the compression ratio in the crankcase.

Bob Patton has had great success with his PCV valve system. It would be interesting to do a dyno. run with a manometer. One nice thing about doing away with Norton's timed breather is you can run the solid cam which doesn't break as easily. The goal is to maintain lower than atmospheric pressure in the crankcase. This keeps the oil inside and inhibits flow upwards past the rings. An open pipe vented to the atmosphere cannot achieve this goal. We need a valve or valves either reed or timed or both. Next trip to L.A. I'll query Ax and if this thread is still alive I'll report his opinions.

Vernon Fueston ( on NOC-L 14th. Jul 1997

Results from the Bob Patton breather modification

Here are the results that I have had with Bob Patton's crankcase vent system.

When I first tried it, I attached some small fittings that I made on my lathe to the intake cover and the manifold of my Mikuni VM34. In the hole of the manifold I at first drilled a .035" hole which I later expanded to .041" for a better vacuum in the system. I then attached the hose from the intake to the manifold, and inserted into the breather as close to the crankcase as possible a Purolator PVC. Result - the leaking slowed from drip, drip, drip to a mere drip: at least 3 times less. But one problem with this system is that it sucks oil out of the head and into the manifold where it goes into the cylinders to be burned, fouling the plugs.

To minimize this effect, I moved the vacuum line from the intake cover to a 'T' fitting located between the PVC and the crankcase. Result - it no longer sucks oil. But there is still that one drip. I checked the system for vacuum, and noticed that it has an ambient negative pressure in the crankcase at revs of 2800-3800, but it varies quite a bit at revs lower than that and then becomes a positive pressure at 4000 r.p.m. and above. I think this is due to the inertia of the valve body, and a lighter valve would probably solve the problem. I am considering making one; both the housing and the diaphragm, on my lathe. The lightest possible diaphragm, and spring will probably solve the problem completely. As it is now, I am on the right track. The last drip might be due to the center head bolt bottoming out. I took it out today and found its threads saturated with oil. I added another washer just to make sure that it torques down right. Will let everyone know if that settles that last drip.

Answering Bob Patton, I think this setup works fairly well, but the trick is getting the right vacuum to the right spot. The orifice is pretty important too; mine is now .041", but it could vary depending on the capacity of the engine. I would imagine that 850s require a greater orifice. Selection of the proper PCV is very important too; it must have good flow, not be constricting, and must react quickly to the pressure vacuum changes in the crankcase. When selecting one, pick one that has openings at least .25" and shake it back and forth to feel the inertia of the valve body. Pick the lightest one, blow through it and suck through it, and find out if it responds well.

When installing, make sure that it is in a vertical position. I found that when mine was horizontal on top of the oil tank, it did not work well, if at all. So, I set it near in the blank spot behind the cam chain chest so that it was vertical. Make sure that the pressure release end is uppermost. This system does work better with an active vacuum from the intake manifold. And in most cases, I think it might solve most leaks. I will be looking for, or making a new fast acting PVC, and will keep everyone informed as to how it works.

Steven R. Schoner ( on NOC-L 15th. Jul 1997

Reliable methods of determining crankcase pressures

The BSA action exemplifies a typical shortcut by not digging to the ground to find the true causes of physical phenomena. In the 50s and 60s, pressure was frequently measured by water columns. This practice had been adopted from the aircraft industry where flow and pressure doesn't vary much with time = quasistatic. However, measuring dynamic pressure such as a rapid varying crankcase pressure, calls for special considerations.

Water has a relatively high density and hence high inertial mass. This affects the so-called resonant frequency, which will be quite low for a water column. In order to have a reading you have to work in the area well below that of the resonant frequency. Above it (2-3 times resonant frequency) the response declines to zero. That's what may have happened at the BSA development stand and may explain the maximum at 2000 r.p.m. I doubt water column manometers will cope with the dynamics of engine (crank) pressure. I could probably do a mathematical proof of this but will leave it for now.

The vital point is to have trustworthy measurements of crankcase pressure with rising revs, i.e., one needs fast pressure transducers and a suitable coupling fluid. There are three types of transducers to choose from:-

  • Displacement-type pressure transducer - the core limit frequency response is 10 Hz (in our case 600 r.p.m.) or less, which makes it unsuitable for engine testing
  • Diaphragm-type pressure transducer - the frequency is limited by the resonant frequency of the diaphragm - but a limit signal frequency in the range 2 - 10 kHz makes it well suited for engine testing
  • Piezoelectric pressure transducer - this has very high resonant frequencies (250 - 500 kHz) and is suitable for high pressures and temperatures. This makes it very suitable for combustion chamber pressure measurements

Recognising that mounted transducers are second-order mass-spring-dashpot systems exited by a frequency-dependant amplitude and a medium varying its density, certain frequency corrections may be necessary either by formula or table even for electrical transducers. I suspect references given by Vernon Fueston in many cases to have a 'traditional' approach to the subject. It would be interesting to learn whether C.R. Axtell has put a serious effort into accurate testing of crankcase pressure and valve dynamics.

I still trust the Matchless valves - light spring loaded PCVs whose resonant frequency is probably in the 200 - 400 Hz range (corresponding to 12000 -24000 r.p.m.). If sufficient interest evolves I could find out exactly how the valve response looks.

Knut Soensteby ( on NOC-L 15th. Jul 1997

The use of manometers for measuring crankcase pressure

I now understand that Vernon Fueston meant that a dial gauge manometer of the self contained type was to be used. Vernon cites an experiment by Dr. Morrison at the University of Glasgow. Be careful when reading this.

The reason Dr. Morrison made the rotary valve to feed the 18 manometers may have been that a quasistatic pressure condition was needed which the manometer could cope with (and which he could take readings from). Although a manometer has a much lighter mechanism and less inertia than a water column gauge I am still sceptical as to how the response when subjected to dynamic pressure will look like. Remember, it's built for quasistatic pressure measurements. I would check this first before using it. If you want to use a manometer up to say 6000 r.p.m., the first resonant frequency should be at 500 Hz or above.

Knut Soensteby ( on NOC-L 16th. Jul 1997

The Purolator valve may not be the best choice

That valve Steve Schoner is using may not be the best choice, in my opinion. A diaphragm or flapper type has a much better chance of keeping up with the pressure cycling. The AV13 seems OK. That there is any net flow of air or oil to the manifold is a sign that the valve has been overwhelmed. I think the right valve will work with the manifold vent in either location, but no reasonable amount of vacuum will sustain negative pressure in the case without a positive seal.

Bob Patton ( on NOC-L 16th. Jul 1997

Why didn't Norton put a PCV into the breather system?

As a minor aside to the thread on PCV and flapper valves, consider the stock Mk.lll breather setup.

The crankcase breathes into the oil tank which reduces the compression ratio considerably. The oil tank is then vented to the relatively low pressure area inside the air filter; indeed lower pressure as the engine revs increase because of the restriction across the filter. The air volume being sucked in by the carburettors is somewhat less than the volume being exhausted by the crankcase, again because of the depression created by the throttles and will depend on the throttle opening.

Mk.llls also used an oil separator that kept a vacuum on the system whenever the pistons were falling and this device will cause fouled plugs if you overfill the oil tank. When the pistons rise, the depression in the air cleaner goes away but the depression in the crankcase increases. Air then gets sucked out of the air cleaner into the tank.

Now suppose we put a fast acting PCV between the tank and the air cleaner? It seems that it would be less troublesome and prone to sucking oil which is often a problem with PCV systems. However, the oil pump relies on atmospheric pressure to feed oil into it, and reducing the atmospheric pressure in the oil tank does not seem like a good idea as it would compromise oil feed to the pump. Instead, I recommend putting the check valve between the tank and the engine, evacuating the crankcase but not the tank.

[Remember that on the stock setup on the 850, the oil tank is slightly below atmospheric pressure anyway because of the breather being plumbed into the air cleaner box. I'm not sure how this would compare with the pressure drop created by including a check valve, but better to be on the safe side. I did have a check valve in the 'open' side of my oil tank breather for a while but it didn't stop the oil leaks either - Robert Smith]

It seems to me that Norton could have kept oil from oozing out of the entire system (tank included) with the simple addition of a PCV, but they had already added so much cost to the Mk.ll, the bean counters were probably tearing their hair out already without this extra cost.

If Knut machines a base for the Matchless valve that fits the hose diameter for the tank vent, I'll try it out and report back.

Thomas H. Allen ( on NOC-L 18th. Jul 1997

More information needed on the crankcase drilling modification

I've been trying to collect as much information as I can during the recent Norton breathing thread but haven't seen one issue addressed. In the technical and service notes there is a modification described that says to drill two 3/8" holes in the timing side crankcase.  Is this modification necessary if the types of valves and all manner of gadgets described in previous posts are to be used? Is the drawing on Page 11 of the technical notes enough to go by? Can someone give me something like a measurement before I drill my crankcase?

It is necessary if one wishes to move the breather to the timing chest; I think I drilled three holes when I did it. It allows the crankcase to breathe into the timing chest. If you have the later 1971 to 1972 breather off the back of the crankcase, you could plumb in any of the one-way valves being discussed to that system without moving the breather to the timing chest if desired. If you have the timed breather off the left side of the camshaft, you have much less need for such a valve. I am not quite sure just why Norton abandoned the timed breather- I suppose they saved some expense, and I guess the cam is stronger for not being hollow. Some indicate that it was too restrictive to allow the volume of air needed to pass. Bringing the timing chest into play adds volume, for a shock absorber effect on the pressures involved, and more theoretical opportunity for the oil to 'rain' out of the air before going up the hose to the oil tank. In practice, I am not sure that it makes that much difference. If I could get a nice reed valve, I might go back to the crankcase breather.

The location of the holes is not rocket science; they are merely holes to let air get from one chamber to another. Locate them as far from the various spindle bosses and supporting webs as possible, so as not to weaken any of same.

Ben English ( on NOC-L 22nd. Jul 1997

More thoughts on the Mk.lll breather & experiences with another PCV

Consider the stock Mk.lll breather setup .....

The crankcase breathes into the oil tank which reduces the [crankcase, not combustion] compression ratio considerably. The oil tank is then vented to the relatively low pressure area inside the air filter; indeed lower pressure as the engine revs increase because of the restriction across the filter. The air volume being sucked in by the carburettors is somewhat less than the volume being exhausted by the crankcase, again because of the depression created by the throttles and will depend on the throttle opening. Mk.llls also used an oil separator that kept a vacuum on the system whenever the pistons were falling (this device will cause fouled plugs if you overfill the oil tank). When the pistons rise the depression in the air cleaner goes away but the depression in the crankcase increases. Air then gets sucked out of the air cleaner into the tank.

The Mk.lll breather plumbing is exactly the same as the 1971 to 1974 setup, except for that extra oil separator. I have one of these in my stash, and have often considered adapting it to my 1972 machine. Do any Mk.lll riders or others have useful reports about the benefits of the separator?

Now, Thomas Allen's idea of a PCV valve in the oil tank breather line; that one is challenging. Maybe I'll try it. For one thing, the pressure variations may not overwhelm the ability of the valve to respond as we suspect happens with most PCV valves in the crankcase hose. "Fast acting" seems to be the hard part. I guess my Fram FV307 PCV valve is too slow. I remounted it so it is vertical, so gravity will tend to close it, but have not given it a severe test yet. I have received my Grainger 9D617 air compressor check valve, but I don't think I like it. It is a ball type (about ¼" ball) which seals tightly when closed (unlike the Fram), but I cannot quite make the ball lift off its seat with lung power. Using a tool to lift it, I can blow through it easily. I think this means it will maintain 5 or 10 p.s.i. in the crankcase, and still won't react fast enough at high revs.

Ben English ( on NOC-L 22nd. Jul 1997

More problems associated with breathers

Maybe I could run the line from the gearbox inspection cover in there too. Right now it goes to the back of the air cleaner. The clutch plates aren't getting all gummed up like they used to before I put it on, but I noticed a little sulphurous crud on them after 5000 miles.

The last time I was timing my bike, I ran it up to 5,000 r.p.m. and one of the inspection covers on the primary case blew off! It was on there pretty tight with new O-rings all around and it leaks a little too. That felt seal isn't supposed to be really airtight in the back is it? Maybe I need to run a vacuum line from the back to a catch bottle, but then maybe it would pull transmission oil into the primary chaincase.

Bob Patton ( on NOC-L 26th. Jul 1997

Description of the AV13 PCV

What is an "AV13" PVC valve?

The item I used was Standard Motor Products AV13. It is a fairly common item at auto parts stores. When you get a look at one you will see that you need to get a threaded adapter to splice it into the hose from the engine to the oil tank. They should have those too. The AV12 is smaller and maybe just as good.

I am going to make some changes to my setup this week. I want to put a catch bottle by the battery and run the vent line from the intake vacuum to the top of it and the run the line from the intake valve cover through the top like a hookah. Never had any sign of oil migrating into the intake tract, but it's not as hygienic as it could be. Maybe I could run the line from the gearbox inspection cover in there too. Right now it goes to the back of the air cleaner. The clutch plates aren't getting all gummed up like they used to before I put it on, but I noticed a little sulphurous crud on them after 5000 miles.

Bob Patton ( on NOC-L 27th. Jul 1997

Old style vs. new style breather setups

Keeping the timed breather and opening the timing chest might provide useful 'shock absorption' to the pressures involved; or, it might have a deleterious effect I can't think of, but someone else will. Whether to close off the timed breather and convert to 850 style, I can't say, as I have virtually no experience with the timed breather motors. If only we could get someone to build and run motors with all variations on a dyno, complete with multiple sensors for oil temperature, crankcase pressure, drag on crankshaft from excess oil (windage) etc.

Norton themselves must have done much work along these lines in the process of making the changes at the time, but I do not recall ever seeing any reports of their results, or convincing explanations for the changes, except for Brian Slark being quoted in the INOA Norton News or the Tech Digest to the effect that the 1972 breather off the back of the crankcase returned more oil to the tank than the oil pump did. But a modification to the oil pickup drillings in 1972 cases, referred to somewhat mysteriously on one of these lists a few months ago, might change that.

If you drill the holes, set up an 850 style breather and assemble the timed breather on the end of the camshaft, you should be able to plug one breather and run the other. That will allow for plenty of experimenting.

Ben English ( on NOC-L 28th. Jul 1997

Do not allow the gearbox to become airtight

It might work, but I would not run a vacuum line to the intake manifold for this one, as the gearbox was not meant to be airtight. There should be a little hole on the inspection cover, and I am sure air leaks past the cable hole too. Too much air would then go into your intake. I think it you were to do this you could make a nice hidden tap off the exhaust pipe just below the gearbox, then run a line to a small fitting on the inspection cover plate; perhaps where the little hole is.

Steven R. Schoner ( on NOC-L 30th. Jul 1997

Some more sources of PCVs

In the unending search for a satisfactory one-way check valve to adapt to the breather, last night I went to:-

  • 1) A Ducati dealer, where a parts clerk showed me their current breather system, which features oil separation but no valve for pressure control. He also mentioned that his father had raced MG Midgets, which had a check valve on a gearbox breather.
  • 2) A power equipment and tractor dealer, where a parts clerk showed me a reed valve used on Tecumseh engines, about 3½" by 1¼", that wouldn't quite fit on the back of my timing cover.
  • 3) An auto parts store, where I was told that one-way valves for power brake vacuum boosters had to be specially ordered. This was inspired by Chris Moog on Brit-Iron, who found such a 'universal' valve in a US chain auto parts store (Carquest) that, plumbed into his Triumph's breather, seemed to reduce his oil leaks.

Ben English ( on NOC-L 31st. Jul 1997

Experiences using a Motormite PCV

Bob Patton's suggested fix using manifold vacuum, and a PCV to evacuate the crankcase seems to be working. The oil leakage has decreased to a mere stain and perhaps a drop every hundred miles or so. But then again, perhaps the PCV ( a Purolator #1020 ) is not quite right.  What is needed is one that responds very quickly, perhaps one that goes on a much smaller engine than auto engines. Maybe, with all the emission laws going into effect, there is a PCV valve that fits a lawnmower engine with characteristics that are closer to the internal dynamics of a Norton engine.

Steve Schoner

As I posted previously, I went to a power equipment and tractor dealer, where a parts clerk showed me a reed valve used on Tecumseh engines, about 3½" by 1¼", that wouldn't quite fit on the back of the timing cover. That shop didn't handle Briggs & Stratton. They did have Honda power products, but there didn't seem to be anything adaptable there either. But I have found something promising. Here is my post of August 11th. from Brit-Iron:-

  • I now have 400 miles using a Motormite (Help! products) 80190 power brake vacuum check valve in my Commando engine breather. (It may be equivalent to a NAPA or Wagner brake part number 89014. I think this is the item mentioned on this list [Brit-Iron] recently by Chris Moog, who used one in his Triumph.) The valve passed the lung power test nicely, being wide open with no restriction under pressure from the big end, and snapping totally shut immediately under pressure from the small side. It was an exceptionally neat installation, replacing the stock 1/2" to 3/8" hose reducer nicely, requiring merely a bit longer 3/8" hose to the oil tank to fall nicely into place behind the air filter.

    This weekend it was put to serious test, as I did 350 miles including some wandering obscure back roads at 20 to 50 m.p.h., but some hours cruising at 65 to 70 m.p.h. and a good 30 miles at or over the 75 m.p.h. and 4500 r.p.m. that seems to severly overtax the breather system.

    My preliminary judgement:- the bike seemed to run a little smoother and stronger, an untrustworthy seat-of-the-pants impression perhaps. But on arriving home, the oil tank was only about 6 fluid ounces down, and levelling the primary case let out only about 1½ fl. oz. Best of all, there is no oil spreading around the crankcase joints, or out of the points cover drain hole, or out of the air cleaner. I can't ever remember getting home from a ride like this with the bike so clean.

Since that report, I have put on another 200 miles, but they have been relatively sedate and thus not a severe test, but she continues to look good.

Ben English ( on NOC-L 21st. Aug 1997

More experiences with a Motormite PCV

I was able to find this part and install it today. The auto PCV (Purolator #1020) seemed to work, but seems to lose its ability to cycle fast enough at higher r.p.m. This Motormite power brake check valve responds much faster because the valve body is considerably lighter than the auto PCVs.

The installation was not as clean due to the 90o bend in it, [by replacing the 3/8" hose from the step down adapter to tank with a longer hose, I placed the valve right at the back of the airbox, so instead of the 1/2" hose off the motor curving and making a 90o bend to meet the adaptor, it comes up straight and the valve makes the 90o bend - Ben English] but once in place it seems to work very well. I think the leak problem has been solved, and my oil use has decreased considerably. I wonder if this Motormite valve will hold up as well as the metal auto PCVs?

Another problem, I hope minor, is that at certain r.p.m.- usually low, the oil tank gets low. This has occurred twice since I installed the check valve and manifold vacuum system. Is the vacuum in the crankcase perhaps inhibiting the oil pump from returning the oil quickly enough to the oil tank?[I doubt that low pressure in the crankcase would seriously impair oil return. Defective oil filters have, twice in my experience. And with the oil in the motor instead of the tank, I had leaks from everywhere, stupendous leaks, as well as rough running - Ben English]. [See also Oil Filters - Editor]

So, Bob Patton's suggested fix using manifold vacuum, and Ben English's suggestion using Motormite's #80190 power brake vacuum check valve has done the trick. There are no detectable leaks, and the power brake vacuum valve has a diaphram that is light enough to cycle with the engine's pulses. It works so well that I have had to oil the cylinder barrel to keep it from rusting! But I am so sick of motor oil, that I use WD-40 instead.

Steven R. Schoner ( on NOC-L 24th. Aug 1997

More experiences with a Motormite PCV

Bob Patton's setup is sound, and works well to stanch the oil from bleeding all over the engine. In private conversation he recommended that I install a "CarQuest DCV1 distributor check valve into the system between the main check valve (installed between the oil tank and the engine so that the air flows out to the oil tank but not back into the engine) and the engine side of the carburettor manifold system. The small DCV1 (also made by Borg Warner as #EC601 distributor check valve) is installed so that the vacuum inside the crankcase does not suck air and gas from the intake manifold when the throttle is opened up during rapid acceleration. This addition to the system has had a marked effect on the throttle response, and the oil does not smell as gassy as before.

Ben English suggested that I use Help Products (Motormite Prod.) power brake check valve #80190. I tried this in place of the much larger AV12 and it not only makes for a much cleaner looking setup, but it seems to work as, well if not better than, the larger AV12 check valve. Inside the #80190 is a small rubber disk that responds very quickly to small pressure/vacuum changes, and it seems to do its job, even at high r.p.m. ranges. The whole setup fits right under the gas tank and seat and you don't even see it unless you look for it. It is a simple, elegant solution to a problem that I am sure affects many Norton twins: I wonder why the engineers at Norton didn't think this solution up? It is just $10 worth of parts and the best $10 anyone could spend to fix the crankcase leaks.

Steven R. Schoner ( on NOC-L 13th. Sep 1997

How many valves and where?

I'm confused, do you have both valves installed? Also, where is the Motormite valve installed? I thought it should be installed between the crankcase and the oil tank, but on my Mk.lll that is a 1" line the entire way and the one side of the valve is too small.

Michael Sullivan ( on NOC-L 15th. Sep 1997

How much oil should come out of the breather?

I have heard that Nortons traditionally puke a little oil out of the engine breather. The story goes even further to say that this is the cause of wet oil filters that leak on the gearbox and convince the bikes owner that he has oil leaks somewhere.  I recently got an e-mail from one of our vintage road racers saying that no oil should come out of the breather. My freshly overhauled '72 Commando put out 30 or 40 drops in a half hours easy ride. Do I have something to worry about?

Paul O'Neil

The only problem is the answer is 'Nobody knows', and 'It depends'. Ideally of course, the answer is 'None at all'.

In reality, any motor with radically varying crankcase volume is going to be pumping plenty of air out the breather. And without a means to ensure that every drop of oil gets right into the return pump pickup, some oil is going to be mixed with that air. Norton obviously expected this, and made various arrangements to separate the oil from the air, first to leave it behind in the crankcase, then in the oil tank. Any that got past that point was supposed to be sucked into the airbox and lubricate the carburettor slides and intake valve stems. The fact that they were continually revising the system indicates that they never achieved complete success. The fact that endless experiments and discussions here and on Brit-Iron take place further establishes that there is no firm answer other than 'As little as possible'.

Ben English ( on NOC-L 19th. Sep 1997

More on crankcase air movements

I am uncertain of how much air is actually going out of the bottom end after the PCV cure. Again, ideally none. The pressure should now vary harmonically between 1.0 bar (14.5 p.s.i.) and 0.905 bar (13.125 p.s.i.) if all crankcase and cylinder joints are leak-free. Except for the few seconds after start-up there should be a negligible stream of gas coming out. First reports have confirmed oil transport with this stream to be unusually low. Due to the low rate of volume/sec breather gas, oil separation is much improved over the previous non-pressure-controlled design as well.

Knut Soensteby ( on NOC-L 20th. Sep 1997

An idea to further lower the crankcase pressure

How does this compare with a setup, such as mine, where the line between the PCV and the crankcase has a vacuum line that goes to the low pressure side of the intake manifold? There is a 0.041" hole in the manifold (Mikuni) and a nipple that screws into a larger diameter hole A hose is attached to this nipple which then attaches to a tee in the crankcase breather line between the crankcase and the PCV. In this hose is another small distributor control valve which allows the intake to suck air out of the crankcase, but not back into it as would occur at more open throttle settings, such as during acceleration when the pressure in the intake would increase. If my reasoning is correct, this setup would induce a much lower crankcase pressure than with just a PCV in the crankcase breather line.

Steven R. Schoner ( on NOC-L 22nd. Sep 1997

Another Motormite PCV conversion

I've been following the discussions on modifying breathers with some interest, especially as a potential method of reducing/eliminating the remaining minor oil leaks on my 1974 850 Roadster. The bike is stock except for an aftermarket air cleaner box which does not have a breather inlet, so I vent my breather to atmosphere (after the oil tank).

I fitted a non-return valve in the line from the oil tank to atmosphere to avoid getting dust sucked back in the oil and I then installed a Motormite 80190 PCV in the breather line from the timing chest to the oil tank. In 200 miles of mixed riding, I would say the oil leaks are maybe no worse and certainly no better. Other than that, the PCV appears to make little or no discernible difference to performance, etc.

Looking for a way of positively venting the crankcase, I wondered whether it might be possible to hook up the breather vent via a PCV to the balance pipe connecting the two intake stubs. That would provide positive crankcase venting, but would it affect performance and starting? In three- and four-cylinder engines, this kind of positive ventilation is done all the time, but with 'our' engines, there's that big rush of wind out of the breather when the pistons come down. If this created positive pressure inside the intakes, the engine would never start of course. Has anyone ever tried this?

Robert Smith ( on NOC-L 23rd. Sep 1997

Linking the PCV into the inlet balance pipe

This [I wondered whether it might be possible to hook up the breather vent via a PCV to the balance pipe connecting the two intake stubs - Robert Smith] is exactly what I did with mine, and the results are much better than just with a PCV alone. Make sure that it is in the line so that crankcase pressure can blow out but not back into the crankcase. Get the tee, and if it is plastic, melt the end that will go into the line between the balance tubes and the crankcase hose and the Motormite check valve. You want to close it almost completely, then drill it out to ream a hole 0.041" in diameter. If the tee you use is metal (a good choice considering the heat) crimp the end so that it closes completely, then drill it out to the required 0.041" in diameter.

I used this setup for quite a while and my engine started up readily, and I noticed no adverse effects, other than a very slight bit of hesitation during rapid accelerations. The reason for the 0.041" hole is that the amount of air going past this orifice is small, but it works to establish a strong vacuum in the crankcase after a few revolutions of the engine. Bob Patton suggested that I put into this line, the one that goes from the balance tube tee, or intake manifold after the carburettor, a small distributor check valve (CarQuest DCV1, or Borg Warner EC601) This valve is inserted into the line so that pressure cannot flow from the intake back into the crankcase during those rapid accelerations when the manifold vacuum decreases to a point where the vacuum might be less than what is in the crankcase. In other words there are times when the manifold pressure is higher than the pressure in the crankcase and this causes air and gas to flow into the crankcase.

I noticed an immediate improvement in that the hesitation upon acceleration vanished. Once you have it all hooked up test it by blowing through the hose that connects directly to the crankcase breather. The valves should be open allowing the air to go out freely. Suck in, and the valves should close stopping all air flow.

Steven R. Schoner ( on NOC-L 23rd. Sep 1997

More crankcase pressure calculations

How does this compare with a setup, such as mine, where the line between the PCV and the crankcase has a vacuum line that goes to the low pressure side of the intake manifold? .....     ..... If my reasoning is correct, this setup would induce a much lower crankcase pressure than with just a PCV in the crankcase breather line.

Steve Schoner

It depends on what you mean by 'much lower'. Manifold pressure is governed by a number of parameters, but you can hope for partial vacuum down to 0.7 bar (10.15 p.s.i.) on average over 180 degrees of crankshaft rotation (t.d.c. to b.d.c. during the induction phase). Noting that you have two induction phases (each 180 degrees or 360 crankshaft degrees apart), the manifold line will be operable in 360 (2*180) out of 720 crankshaft rotation angle degrees. As you note, large throttle openings will increase the manifold pressure (i.e., partial vacuum) closer to ambient. To some degree, low manifold pressure overlaps low crankcase pressure, particularly with one of the pistons at the induction stroke close to t.d.c.

However, with crankcase pressure otherwise rising while the pistons are travelling towards b.d.c., you may be able to maintain or even lower the crankcase pressure to values below 0.905 bar (13.125 p.s.i.). The downside of this balance is that if there's a large pressure difference between manifold and crankcase (absolute pressure in manifold being lower), a sizeable quantity of crankcase gas may flow by, thereby upsetting the carburation. This could easily happen with a worn engine which will have gas leaks into the crankcase. Therefore you don't want too much pressure deviation over the system unless you monitor the evacuation system frequently. Further, there is the question as to whether crankshaft oil seals are able to withstand a large pressure difference of up to 0.3 bar (4.35 p.s.i.). They may deform permanently, thereby making it impossible ever to maintain a pressure below ambient. Owners wanting to experiment with Steven's setup should bear in mind that with crankcase pressure well below ambient, there will be a net power loss due to increased piston pump work of the gas displacement process.

Knut Soensteby ( on NOC-L 24th. Sep 1997

Why should Steve Schoner's setup result in a power loss?

I fail to see why this [there will be a net power loss due to increased piston pump work - Knut Soensteby] would be true. Once the pistons pump the air out, the check valve keeps it from returning. Manifold pressure is lowest when the slides are closed and the engine is not producing much power or blow-by either. If the crankcase pressure is very low it will suck the pistons down with the same force that is required for the crank to push them up.

Anyone who is experienced with riding a two stroke with a compression release or driving a truck with a 'jake brake' knows that the maximum drag is caused by forcing the air in and out through a restricted opening. When checking a two stroke for seal leakage it is common to pull 10" of mercury to check for leakage. I have never experienced seal damage during these tests.

Vernon Fueston ( on NOC-L 24th. Sep 1997

No power loss with the Steve Schoner setup

Is not my setup, the basic one recommended earlier? by Knut Soensteby? [I did recommend fitment of a pressure control valve, but none hooked to the inlet manifold for the reasons stated previously - Knut Soensteby]. The only change is the distributor control valve. With the addition of the control valve into the line from the intake manifold the throttle response returned to what it was before the breather system was installed. As for the said loss of power-- I have not noticed it, and every joule of power lost would certainly be noticed at my elevation of 7000 ft. If anything, the overall performance of my engine has improved. It burns much less oil (the crankcase vacuum probably helping in oil control), and it runs much smoother and cleaner than before. I've gone over thousand miles on this breather system thus far. Not a single drop of oil has stained my engine since, nor have I noticed any problems with regards to engine seals. I really doubt that a negative pressure would distort the seal surface anyway. These seals are pretty tough, if anything minute amounts of air might creep past the seal surface and into the engine, especially if the seal is worn. But as for damage, I doubt it. If so, it would certainly have shown up earlier. Perhaps, Ben English and Bob Patton, who have similar installations can shed some light on this subject; in fact it was their systems that gave me the idea.

Steven R. Schoner ( on NOC-L 23rd. Sep 1997

No power loss with Steve Schoner's set up after all

Long-term behaviour of the seals remains to be seen. As for possible distortion of oil seals, it should be noted they are not made to act like pressure seals. Having said that, 0.3 bar pressure difference equals 3 or 0.03 and I believe the seal can cope with this load but don't quote me (3 is about the weight of a medium sized glass of water applied through your index finger.) If not, it is possible to reverse the lipped seals of the mainshaft and on the timing side to allow ambient pressure to close the seals, not to open them up. Without reversing the drive side seal, it's suggested not to run a dry primary drive due to possible contamination of pressure leaks. For a firm opinion on the effect of venting the crankcase with the assist of manifold vacuum, pressure measurements with (fast) piezo-electrical transmitters need to be taken.

I fail to see why this would be true. Once the pistons pump the air out, the check valve keeps it from returning. Manifold pressure is lowest when the slides are closed and the engine is not producing much power or blow-by either. If the crankcase pressure is very low it will suck the pistons down with the same force that is required for the crank to push them up.

Vernon Fueston

Vernon is right, I have to apologize to all - I made a serious error of thought; sorry about that. There will be some changes within each phase of the displacement process (the exhaust phase will require more energy, the intake phase less or even dissipate energy, the first part of the compression stroke will require more energy) but the net outcome is zero.

Checking up on the blow-by phenomenon, only about 1% of the cylinder charge leaks into the crankcase. With the standard setup, crankcase blowby gases vents to directly to the atmosphere and constitutes a source of HC emissions. With the modified system, crankcase is vented to the intake system and blow-by gases are in fact recycled. So you actually do the environment a favour with the manifold vent.

Knut Soensteby ( on NOC-L 24th. Sep 1997

Power increases with vacuum

This a an extract from Bill Jenkins' text on engine building:-

"...At peak power on the dyno, this sytem produces 4-5 inches (mercury) of vacuum. ...According to to dyno tests by Bill Million, there is a definite power advantage when the case vacuum goes above 55" to 60" (water)."

I don't know the conversion factor of pressure measured in inches of water vs. mercury, but the data they gathered seemed to indicate power increased with vacuum. Reduced aeration of oil and increased oil scavenging in the case along with reduction of oil contamination of the combustion chamber are possible explanations according to Jenkins.

Bob Patton ( on NOC-L 24th. Sep 1997

Power increase with vacuum - some calculations & not convinced

4-5" in mercury (Hg) corresponds to0 .135-0.169 bar (1.95-2.45 p.s.i.) 55-60" water column corresponds to 0.137-0 .150 bar (1.98-2.175 p.s.i.) This means in the experiment they had 0.865-0.831 bar of static pressure at peak power. In the second experiment a crankcase total pressure of 0.863-0.850 bar or below was found to give a power advantage.

Some further values can be given (source:- John B. Heywood: Internal Combustion Engine Fundamentals; McGraw Hill). In a typical 5 litre V8 engine, manifold static pressure at full throttle is 0.827 bar (11.98 p.s.i.), at idle; a value of 0.490 bar (7.1 p.s.i.) can be extracted from their data. (Fig. 7-24, p. 311) The previous value corresponds with Jenkin's data. Idle and low r.p.m. pressure is very low and makes me worry about the seals if this pressure were to be induced in the crankcase. Further, there is a significant variation in manifold pressure, making me think that some kind of vacuum limitation might be needed in order to keep gas flow out of the crankcase within limits.

The hypothesis by Jenkins to explain the power increase doesn't quite convince me. To some degree oil will be affected by the gas swirl, but I assume gravity and mechanical movement to be mainly responsible. The latter factors are of course not affected by a reduction in crankcase pressure. Rather than oil, I assume a reduction in work executed due to polytropic compression of crankcase gas, a reduction in gas swirl as well as a reduction in heat radiation may explain most of the power increase. It would be interesting to know how much power increase was actually measured by Bill Million. I'd be surprised to see more than 2.5% on average over the rev range.

Knut Soensteby ( on NOC-L 25th. Sep 1997

Can you use crankcase pressure to supercharge the engine?

Has anyone tried the next step in this idea, using the crankcase pressure to supercharge the engine? I saw an article about this being done on a racing Triumph a year or two ago in one of the classic bike magazines.

John Woodgate ( on NOC-L 28th. Sep 1997

Supercharging with crankcase pressure - difficult

There are a lot of problems in implementing this. At first it seems like a great way to get a two to one supercharger for free. However there is a problem in how to keep the oil in the crankcase out of the inlet flow, then there is the fact that we do not use pressure carburettors. If we force air into the carburettor mouth, we will blow the fuel back into the float bowl unless we also pressurize it. Even if we do that, we will blow the slides to the top unless we pressurize them too - instant full throttle as long as the engine runs. We could put the carburettor on the crankcase, two-stroke style, but then we have our fuel getting mixed with the engine oil. If we overcome all of this we still have to plumb the crankcase into the inlet in such a way as to retain good flow without adding so much volume that we lose the crankcase compression that we are relying on to boost the performance. I hesitate to say it can't be done, but there are some formidable hurdles in the way.

Vernon Fueston ( on NOC-L 28th. Sep 1997

Supercharging with crankcase pressure - an attempt

In his book, "Rolling Thunder - a history of the BSA-based CCM four-strokes", Bill Lawless describes how CCM boss Alan Clews tried just this set up, which he called a 'five-stroke', in the late seventies as a desperate gamble to keep the ageing OHV four-strokes in the same power league as the two-strokes. Britain's National Research & Development Council funded the project. Design details included a plenum chamber to even out the pressure pulses coming from the crankcase.

I don't believe it ever saw the light of day in competition, although exactly what problems arose isn't discussed in the book. Supercharging with carburettors is done, of course, but the carburettor is usually on the negative pressure side of the charger. That means stuffing air-fuel mixture through the crankcases with all the extra considerations implied, including really good seals. Of course, there's always fuel injection, but then we're getting into even deeper territory. An interesting concept, but like water injection, it seems destined never to be fully developed.

Robert Smith ( on NOC-L 30th. Sep 1997

An alternative to the Motormite PCV

I've been poking around for the Motormite checkvalve for a few weeks and finally got serious about finding one yesterday. I did find it, cheap at $3.49 US. As previously mentioned its main drawbacks are the 90 degree bend and that different tube sizes are used for inlet and outlet. Since there is very little space in this area of my Commando, I thought I'd look for one that is straight and doesn't require tubing adaptors to match the size. I found it at the local VW hot rod shop under VW Part No. 191 611 933 at a price of $14.00 US. Here's how to do the installation; it all took less than 5 minutes and I didn't get dirty:-

  • 1. Remove the breather hose from the tank filler neck
  • 2. Pull up about 2 inches of slack
  • 3. Cut 3 inches off hose
  • 4. Slip the checkvalve into the respective hoses
  • 5. Slip it back together
  • 6. Secure with lock wire or hose clamp

I took a leisurely trip of about 120 miles after the installation, with lots of high r.p.m. full throttle blasts. and some time at a steady 70 m.p.h. Oil had been leaking from the front cylinder head studs at a pretty messy rate but anyhow, I cleaned it up with contact cleaner and there was no sign of leakage. So, preliminary results are good and by the way, the tick-over is much more stable and smoother.

Thomas H. Allen ( on NOC-L 2nd. Oct 1997

The Krank Vent Plus

It's been a while since I've seen discussion on this subject, but I just saw one of these [PCV] valves being advertised specifically as an aftermarket item for motorcycles. It was in an 'American Iron' magazine, manufactured by Hayden Co. (the oil cooler people?) and called the Krank Vent Plus. It's marketed as a Harley item but could be just as well installed on a British bike. It's of in-line design, looks like a metal fuel filter and has push-on hose connections. The advertisement claims 2 to 4 h.p. increase due to negative case pressure, and a reduction in blown or leaking gaskets. The suggested retail is $109 US, but it can be had for $99 US. Does anyone have experience with these?

Bill Larson ( on NOC-L 5th. Dec 1997

  • My $3.00 Motormite product vacuum brake control valve works just fine - Steve Schoner
  • Only $100 for a $9.95 part, what a deal. Its nothing more than a PCV from an auto application - Ken Dubey
  • The VW part works just fine and costs only $14 US - Thomas Allen

Can PCVs be used on Dominators?

I've been following the crankcase breather thread with some interest, but most of it seems to be linked to Commandos. How does it apply to other Nortons? My 600cc 1960 '99' has a so called 'timed breather' at the top of the rear of the crankcase near to the primary drive chain case. As far as I understand it, this breather was originally vented via a tube to the drive chain as an oiler. However, like many others, I've diverted the breather pipe to a catch-pot, under the rear mudguard in my case. I collect about 25 ml of oil per 100 miles - and dispose of it. I oil the chain frequently by other means, and I don't now have oil getting on to the rear tyre.

What I want to know is - does the thread about crankcase breather valves apply to the Dominator '99'? Is there one of these valves I can put in-line in the breather pipe with a view to eliminating my catch-pot (and at the same time reducing the amount of oil breathed over), and being able to run the modified breather pipe either into the the oil tank or directly back into the vacuum side of the carburettor? In the case of the latter, any suggestions on where to drill for the tapping to take the pipe? Do these valves actually reduce the volume of oil 'breathed' out of the crank-case? If so, by how much, and can someone explain in layman's terms how they do this? I understand the stuff about negative crankcase pressures, and thus the reduced tendency for the case to leak oil, by virtue of the vacuum line to the carburettor, but my crankcase is already pretty oil tight. What I want to do is eliminate the oil carry-over, and get rid of my catch-pot. If the modified breather is fed back into the carburettor, has anyone any idea of just how much oil might get pulled into the cylinders to potentially oil up the plugs?

P.E. Preece ( on NOC-L 11th. Dec 1997

Do not route modified breathers back into the carburettors

For venting the crankcase by manifold suction, this method is not recommended. I discussed the venting subject with the honourable Mr. Les Emery of Fair Spares and he indicates that scavenging by the gear pump (6 start version in this case) will eventually fail due to the suction being too low to overcome the low crankcase pressure. (With a 3-start pump the problems are likely to be reinforced.) The result is overfilling of the crankcase, increased oil drag, increased oil consumption, carbon build-up, a smoking exhaust, new leakages and, in the worst case, a starving top-end and increased wear of the crankshaft big-end. The actual pressure established by manifold vacuum is around 0.58 bar on the 750cc Commando @ 6000 r.p.m. (I had forecasted 0.65-0.7 bar), as opposed to a constant 0.89 bar (0.91 bar theoretically as shown by the undersigned) when using the PCV approach.

Fitting a PCV is a modest but effective solution. Constant partial vacuum at all revs is an asset. It will render your Norton engine practically oil-tight, provided all seals and gaskets are fine. Needless to say, Les Emery now fits a PCV to every new Norton he (re-)makes. During the past discussion of the subject I made an estimate of the vent gas transport with and without using a PCV. The quantity of oil transport is significantly reduced, due to gas flowing in quantity during the startup phase only. If revs are kept low for the first 30 seconds, the effect of droplets being carried away by gas is minimized. Later on, when engine gets warm, oily fog will pass through the PCV but at a much reduced level. By thoughtful design it is possible to separate the droplets at the vent stub entrance (the 850cc crankcase is advantageous in this respect, as well as the Atlas/G15 engines, less so the 600/650's). The reasons for a continuous gas stream are leaks in the cylinder area (combustion gases) and at the seals (top end, crankshaft). This is one reason why the engine should always be kept in good shape.

Knut Soensteby ( on NOC-L 12th. Dec 1997

What is so wrong with the original setup anyway?

According to Bacon, all Norton big twins used the same breather up to 1971. This was a timed system driven off the end of the cam. Triumph used a similar system until 1969. In my opinion, this is a good system giving little or no trouble and holding a below ambient pressure in the case. Norton then went to a hose from the crankcase to the oil tank with no valve and breathed the oil tank into the air cleaner to pull out the fumes. In order to get the crankcase back to the lower pressure it enjoyed with the timed system, many owners have placed a check valve in the hose so the air cannot flow back into the case on the upstroke. There are many variations of valves and plumbing, each with its proponents. I have seen oil tight engines with these systems, but since you already have one why 'fix it'? As the engine wears you may get some oil out of the breather, which is why Norton routed the timed breather hose through the oil tank on the Commandos.

Vernon Fueston ( on NOC-L 12th. Dec 1997

Some oil will always come out of the breather

The problem of oil blowing out of the breather is a separate problem from evacuating the crankcase to reduce oil leakage. Oil will still come from the breather; how much depends on the model, Combat Commandos being the worst. I have worked this problem on my G80CS Matchless for a couple of years and adding an additional breather with a check valve reduces the problem but doesn't eliminate it.

My next project will be to make an oil separator that will drain the oil back to the tank and vent the air to the atmosphere. In the case of the G80CS, returning the breather to the tank simply changes the location of the oil leakage to the cap of the oil tank, from where it dribbles down the side of the tank, only to be wiped off by my pant leg.

Thomas H. Allen ( on NOC-L 12th. Dec 1997

A check that PCVs do not adversely affect oil levels

I have incorporated a combination system where a PCV valve is installed between the oil tank and the crankcase. Then, between the PCV and the crankcase I have a tee which then runs a line to a tap into the carburettor manifold (low pressure side). This give the additional vacuum boost that you speak of.

Early in this project, I too thought of the ramifications of oil scavenging, and tested the system to see if the pump was indeed being impaired in this vital function. The manual calls for 200cc of oil to be present in the crankcase after all the excess oil has been returned to the oil tank. In practice this varies somewhat, it can go as low as 150cc. I ran the engine at idle for several minutes, turned it off, then drained and measured the amount of oil present - 200cc. Then I put the oil back into the oil tank, restarted the engine (drain plug in), ran it up to 3500 r.p.m. for a minute or so, turned it off and drained the crankcase again. Result - 170cc of oil present in the crankcase. At higher r.p.m. the result was the same.

On the road tests, I would go 10 miles or so, up hill, down hill, straight, stop and check the oil in the oil tank. Having noted the level before hand, it did not change to any great degree. These tests were done 3,000 miles ago, and my engine runs fine, with not a drop of oil on the head or anywhere else. I doubt that the manifold vacuum boost will adversely affect the scavenging ability of the oil pump, unless the pump is bad or worn in the first place.

Steven R. Schoner ( on NOC-L 14th. Dec 1997

Possible consequences of having too much crankcase vacuum

I think Knut Soensteby is right about the possibility of having too much crankcase vacuum. However, the repercussions mentioned haven't materialized in my experience. Maybe we can't generate the type of vacuum required for those problems in spite of our best efforts. Some less dramatic consequences I suspect but can't verify are wear at the tachometer drive and intake valves due to the absence of lubrication. The pressure gradient near these seals might be keeping the oil away from these boundaries, if you will, especially if you have the seal installed on the tachometer drive which seals off the oil from the shaft.

The oil leaks that cause the most people grief on the Commando are from the head. Two important issues seem to stand out:-

  • The crankcase and the interior of the head are two distinct spaces
  • The head was designed for low volume, low pressure lubrication

Even if the net pressure in the crankcase is never far from atmospheric, that doesn't mean the same thing applies to the head. The oil return passage and the grooves in the front of the tappets are the only way out of the head: not much. I would really like to put a gauge that spans vacuum to pressure on a head without a PVC and run the engine up to 6000 r.p.m. to see what happens. I broke my engine in slowly last winter keeping it under 4000 r.p.m. and it stayed dry. The first time I ran it to 5000 r.p.m. it puked oil all over out the head gasket. There's some serious pressure going on in there.

Bob Patton ( on NOC-L 21st. Dec 1997

Over-pressure in the cylinder head - some thoughts

For obvious reasons, I can't comment on Bob Patton's engine. The scavenging problem obviously did occur to Mr. Emery's engine(s), which I have reason to presume were in top condition. I hope being able to go after the truth when I get my first pair of engines (20M3s) on a test stand by the end of February 1998. Your remark on a positive pressure gradient at the top of the intake valve guide could be a problem, but I'd have to investigate further to make a serious comment.

The following discussion assumes a 20M3/20M3S 750cc engine with 6 start oil pump, pressure-lubricated cylinder head with rocker shafts fitted the proper way, and an unrestricted oil drain bore. At the outset I note that over-oiling of the cylinder head was known to Norton, as they fitted new followers from engine 151175 to increase oil drain from the head (information by USNOA). The modification consisted of increasing the chamfer on the followers. Whether this worked I do not know; I guess not but it may have improved lubrication of the cam/follower contact area. There is no new part number to substantiate the alleged design change. Is Bob's assertion really correct?

To be sure, I examined my barrels and heads again (all off 20M3 engines) and I noted they all have an oil drain to the timing side rear which is where I suspect most oil is drained. Until today I thought this bore to be responsible for pressure equivalencing as well but it might be too small. Now with the oil drain and still an unduly high pressure I can only speculate. If there is a quasistatic pressure build-up in the head above the level of crankcase pressure, this can only be possible by restricting the drain severely (by oil impurities). The drain ends in the vicinity of the right hand inlet valve and is thus running cool (further downwards it gets hotter). Formation of oil/steam bubbles which could block the drain seems unlikely at first but at second thought might be a real possibility if oil drains too slowly and is allowed to heat up.

Other causes could be saturation, i.e., oil not draining by the same amount entering the head. On the other hand, if there is no quasistatic pressure but rather a dynamic pressure of higher frequencies, the oil drain could be blocked by dynamic pressurization from both ends; rising crankcase pressure with peaks slightly above ambient (without a PCV fitted) and a top-end pressure with peaks also above ambient (otherwise the head wouldn't leak). Such a situation could reinforce the problem by allowing an oil build-up. In the end oil would squeeze out everywhere.

I find it the most interesting to find the primary cause of top-end pressurization. Is it the oil circuit? Does combustion pressure and a porous casting cause exhaust gases to leak into the 'rocker box' volume? Is it a heat problem, as indicated above? The true explanation could even be a combination of these possibilities, or another effect which hasn't been accounted for as yet.

Knut Soensteby ( on NOC-L 12th. Dec 1997

Some questions on Norton's breather design

I replaced the reducer in my 1973 850's breather line with the Motormite Brake Booster Valve. I can't tell if the engine revs any freer, it certainly hasn't hurt the engine it that respect, though. I wonder how durable is this item likely to be, it probably isn't designed to operate in an atmosphere full oil mist and blowby. Hopefully it won't fail in the shut position.

As far as oil tightness it seems to have totally eliminated my engine's weeping. This raises some questions-

  • Were the earlier 750s with the timed breather more oil tight (all other things being equal) than the 850s with the 'passive' breather?
  • Why did Norton do away with the timed breather via the cam shaft?
  • Were camshaft failures using the hollow camshaft common?
  • How did the air get into the hollow camshaft on these, there must be a hole or holes between the lobe?
  • How did the timed breather on the back of the crankcases work?
    Was it run off of the timing chain, the oil pump, or something like that?
  • What was the main problem with this style breather having more oil returning via the breather than via the oil pump?
  • Would this damage the oil pump, cause more oil leaks due to oil mist?

I understand these engines also had problems with the oil pickup in the crankcases being mislocated causing wet sumping under sustained high rpm use.

Eric Goforth ( on NOC-L 18th. Jan 1998

Durability of Motormite valves

I think that Motormite valves are more durable than they look. What is inside is a oil resistant rubber disk that is light enough to respond to the pulses of the engine. I installed one 3000 miles ago and it is working fine. I think if it fails it will just allow air to go through it in both directions, as if you had no valve there at all.

These Motormite valves are much better than PCVs as the valve body in the PCV is usually a steel plunger weighing several grams or more. At the low rpms, and big volume displacements of air in auto engines, they work well, but for our motorcycles-- no. The valve body does not respond fast enough because of its inertia, especially at high rpm. The Motormite valve is simple in construction, and I have yet to hear of one failing.

Steven R. Schoner ( on NOC-L 18th. Jan 1998

An idea for a very simple solution

The discussion on breathers prompts this thought: surely one approach is to do away with all clattering valves, pressurised tanks etc, and just connect a long plain pipe from the crankcase to, say, exit near the number plate as on late Triumph twins. When the pistons are on the downward stroke, crankcase pressure will push air out. On the upward stroke, a vacuum will be maintained in the crankcase because outside atmospheric pressure will be less powerful, and will stay that way, helped by friction in the pipe. It's simple and tidy.

Jordan Princip ( on NOC-L 7th. Feb 1998

Another solution

Here's what ultimately worked. This top end was last on a 1971 timed breather lower end, and it leaked oil at any engine speed above 3500 r.p.m. This I attributed to the engine having 103,000 miles on it as almost everything inside was bin material. I was quite surprised then when the reworked top end, now on a 1973 lower end, leaked 10 times worse than it did before. The solution was a matter of lower end breathing ability.

Neither the timed breather or the hole the case breather of 1972/73 is really up to the job. The timed system works somewhat better in that the air that does get out, can't get back in, so there is less residual air being pumped around inside the motor. My particular problem was the result of a warped cylinder head (in the pushrod tunnel area). The oil returning down the pushrod tunnels was meeting air being pumped up the tunnels, the result being that both were finding the path of least resistance to the atmosphere, out under the composite head gasket in the three bolt area around the tunnels. If you take a good look at a disassembled top end, you'll see that there's really very little sealing area around the tunnel area, so it isn't difficult for a good blast of high pressure air to find its way out through there, especially if the head isn't perfectly flat.

What I did to temporarily cure the leak was a system that there has been much discussion about. Basically, I used a Hastings HV172 PCV in the breather line from the crankcase to the oil tank. This particular valve has good sealing action in the reverse flow direction, and has exactly the right fittings to replace the factory reducer just before the oil tank. This cured most of the leakage problem, but there was still the problem of a moderate leakage problem at higher r.p.m. The final bit of the solution, involved drilling a ½" hole in the intake rocker cover half way between the holding nut and the bottom of the cover, to allow relief of any pressure buildup in the top end. Into the hole I threaded a 3/8" x ¼" barbed hose fitting, and a piece of 3/8" fuel line going straight back between the carbs.

When I first started the engine with this line open, there was a fair amount of air being pumped out initially, and it gradually fell to nothing as the PCV did its job. The remaining work to be done is to tee this line into the breather line to the oil tank. I'm planning on trying it on both sides of the PCV position in the line to see if one way works better than the other. After ~100 miles of running with just a filter on the end of the rocker vent, there is minimal oil leakage from the head, so now at least I can run the bike until it's time to put it up for the winter. The real fix will come when I remove the head and have it resurfaced so it will seal.

Ken Dubey ( on NOC-L 7th. Oct 1998

Another very simple solution

I have a 1969 Commando and was always having a problem with oil leaks for no apparent reason, especially out of the tacho drive. Previous to my ownership, the crankcase breather had been moved from the end of the camshaft, this hole being blocked up with a bolt, and put down the bottom at the rear of the crankcases as on the later Combats. I tried all the ideas on the tacho drive; most of them worked for a while, but then the dreaded leak was back.

I drilled and tapped the rear rocker (inlet) cover, screwed in an air hose fitting and clamped a piece of clear plastic tubing to it. I then teed this into the breather hose between the bottom of the cases and the oil tank. I used the clear tubing because I thought it would be interesting to observe oil flow if any.

My theory is that the top end of the motor becomes pressurised and retains that pressure which encourages the oil leaks. Another pipe hooked onto the engine like this simply means another breather or more breathers or better breathing, whichever way you want to look at it. The more breathing the better!! Now I don't have oil leaks from the top end or the tacho drive or anywhere else for that matter. I like to trundle along at a good 70 m.p.h. when I can, keeping her well up on the top side of 4000 r.p.m. There is some oil flow visible through the tube, but there seem to be no problems associated with this.

I run the final tank breather to the ground and get no puddles under the bike or oil spray over the rear as I ride. Drilling the rocker cover means no major modifications to anywhere on the engine, only the rocker cover. I fitted the biggest fitting I could into it which meant the hose size is similar to that which comes off the regular breather. If you still have the breather on the end of the camshaft, it would mean simply teeing into that line. I did no other modification to the breathing system at all and it has meant no more leaks. If you were to do it and it didn't work or you were not happy with it, the worst you have done is wrecked a rocker cover.

Bob Davis ( on NOC-L 17th. Jun 1999

Success with the simple breather modification

I have also taken this advice and tried it on my 1972 Combat with good results. I found it easier to seal the rocker covers (outlet) by venting the inlet. I also run the breather out of the oil tank along the left side of the frame and then under the fender. You cannot even see it and I do not get any oil leaking from it after a long ride.

Michael Guidera ( on NOC-L 17th. Jun 1999



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