4S camshaft

Unles you are always going to keep the revs above 5,000 the standard camshaft is pretty good. The loss of flexibility at low revs of the 4S is a price I wouldn't want to pay. But that's just me. I love the bottom end grunt of the standard 750 engine and find it does go well enough (rather well really) once you reach 5,000 rpm on the standard camshaft. I also found the standard camshaft well suited to twisty short-circuit race tracks...

The biggest problem with a 4S cam is the clatter caused by the recommended .016" valve clearance.

I fitted a 4S cam to my 850 a few years ago, and had the head gas flowed at the same time. I expected to see a bit of extra go at the top end at the expense of bottom end push. It was actually a win-win situation, being better all round. The fuel consumption also improved (dropped) at the same time. Of course I don't know how much of this is down to the head work. (You generally need to weaken the mixture a bit after gas flowing).

I was advised by somebody I trust very much to set the valve clearences at 12 thou, for quieter running, or 16 thou for a bit more performance. I've alwaysrun them at 12 thou, although recently learned that this can quite safely be reduced to 11 thou.

Cheers

Jeff

I used 4s cams for racing and won a lots of races on them. Good all round power not just top end. I have also used them on the road with the same result.

However the std cam is very good and unless you plan to ride at licence losing speed or have a 4s cam or do track days just put the std Commando cam back in.

Cheers Tony

I have a 4S camshaft in my 650SS motor, but, as Colin expressed, the clatter with .016" valve clearances offends my mechanical sensitivity, or sensibility; such as it is!

Jeff mentioned about running them at .011"-.012", but is this recommended, or does it have any drawbacks? Can anyone explain to a simpleton like me why the clearances have to be so vast?

I am about to do a 500-mile tighten-down prior to letting her rip; well, pottering probably; up the hill at Prescott this weekend, and it is an opportunity to re-set the clearances.

BTW, I have found bottom-end grunt to be very good with this cam, in the limited mileage it has done thus far, so no complaints there.

Cheers. Ian

.......nobody.......?

Previously ian_cordes wrote:

.......nobody.......?

Valve clearance refers to the small gap betweenthe valve lifter andthe valve stem that ensures that the valve completely closes. On engines such as ourswith mechanical valve adjustment, The clearance serves two functions, one is to allow the valve to close fully and the second is to allow a space for thermal expansion to take place as the engine reaches operating temperature. If the gap is too small then as the various component parts expand with temperature the gap would be lost and the tappet adjuster on the end of the rocker arm, would bear on the valve stem causingthe valveto be depressed and remain slightly open causing burning of the seat and probable overheating of the valve. Conversely, if the gap is too large it generates a lot of mechanical clatter and alsothe valve may not be fully opened at maximum lift resulting in a loss of potential performance. I have fitted 4s camshafts into four Commando engines(both 750 and 850) all used for road use, and find that they give improved performance across the rev range. I set my valve clearance at 11 thou inlet and 12 thou exhaust and find this gives a good compromise between performance and clatter. I would only reccomend setting at 16 thou if the engine was to be used in a full race bike with the consequent potential higher operating temperature.Typically, the clearancesshould be readjustedevery 20,000 miles (32,000 km) or after tightening down the cylinder head bolts.

There are another couple of points to watch for if fitting a 4s or other high lift/ long duration camshaft. The first is to ensure during assembly of the crankcases that there is sufficient clearance tn the camshaft tunnel to allow for it to revolve even when expanded at full operating temperature. The second point is to be mindful of the effect on valve /rocker arm / pushrod geometry if you are using a high compression skimmed head as in the Combat engine or if your standard head has been skimmed down at any time. If you are concerned that the geometry may be too acute then shortened push-rods are available from suppliers of Norton engine tuning parts like Norvil or Mick Hemmings. Hope this helps !

Alan Clarke Shenstone branch sec.

Alan. Thank you for the reply. I fully understand theconcept ofvalve clearances. Having rebuilt quite a few engines in my time, it would be rather lame of me not to!That wasn't quite what I asked.However, Iam a loss to understand how .016" can be taken up by engine temperature.The recommendedclearances for the combat engine and the production racer, for example,are only abouthalf that quoted for the 4S cam, at .008" inlet, .010" exhaust, when using the triple Scamshaft, and that for racing conditions.

My 650SS will not be raced, and is for road use, so I will take your recommendation, and re-adjust the clearances.

When I built this engine, I opened out the camshaft tunnel to ensure there is sufficient clearance, as you mentioned. The head has not been skimmed, and I also checked the valve/piston clearance, as well as ensuring that the valve springs are not coil-bound at full valve opening.

So it should all be ok, and a bit quieter when I re-adjust the clearances.

Out of interest, are these revised clearances something you have found by experiment, or are they recommended as a road-use setting?

Ian

I read that this is a polydyne cam and that it was designed to use these clearances. changing the clearances will affect the cam timings and the point on the cam which meets the follower will not be where the design intended.What this all means is that you are on your own.

Robert. I believe that is the case, but whatevera cam design, I guess it is a given that they work on specific clearances. Question is, would those clearances vary, depending on useage, as Jeff and Alan are suggesting? When racing, the higher part of the rev range is constantly in use, whereas for road riding, that is not the case.

I would not be on my own if I reduced the clearances, I would have Jeff and Alan with me! However, you can probably tell that I have some reservations about changing the recommended settings, but feel that, if the clearance is not being taken up in normal use, then the hammering of the valve train can't be doing any good.

Ian

Hello Ian, I've just spotted the thread again. I seem to have started a debate and then disapeared for a while.

I fully understand the risks of burning valves out, and realise that by reducing clearences the lift and duration will be increased slightly. Like you I am usually a bit cautious but I obviously trusted the people who advised me. I've not seen anything in writting to back up the claims that it is safe to run at these reduced clearences, like you say, we're on our own. I hope Bob Johnson doesn't mind me quoting his name, but if I recall correctly he was one of the people who told me he has run reduced clearences in the past. He rides a lot further and faster than I do, so if he's not had a problem, I doubt I will.

Saying that, I have had to make a few motorway journeys of 150 to 200 miles with the cam in, and like to rev it out now and again. I also nearly melted the engine keeping it running for too long in an enclosure waiting to get on to the track for a parade at Donnington one Norton Day, so it's not all gentle use.

I have to admit I've never tried running with 16 thou clearences, perhaps it would run even better?

Good luck with whatever you decide to do.

Cheers, Jeff

Hi Jeff.

I did a search on 4S cams, and came up with your thread!

I am of a mind to try at the reduced clearances, and see if there is any noticeable 'seat of the pants', as well as audible, difference.

I did try and attach a short video taken from the bike, whilst queuing for the start line at Prescott on Sunday, when the clatter is clearly audible. The file was too big, apparently, and it did not let me attach it.

It just seemed to me that .016" is a huge clearance, but I have since read that short stroke Manx Nortons run an exhaust valve clearance of .028", so I don't feel so bad .... having said that, the inlet is only .008".....

Ian

Hi, I've run a 4s in my fastback for at least25 years, possibly longer, It has toured France and Holland, been my mount to approximately 20 visits to (spectate) at the IoM. It has been down at each of the proper bends (i.e.not chicanes)at Mallory and even won a couple of CRMC races there when they ran production classes. I must admit that I always thought that the clearances were 8 & 10thou. Low down toroque is better than my very standard 850, it pulls lovely from 1500 revs in top, and picks up its skirts and gets exciting above 5,000 revs - just needs a lot more loving care and attention than my 850. I've even been out for a 180 mile pootle round Staffs, Warks, Shrops on it today. However,in my older age, I have realised that if I really want to go faster the most effective, most reliable and cheapest way is to buy some thing modern - an old Norton or whatever can't hope to match it. I ride my standard 850 much more often because it is far less tempermental than a bike with all the go faster goodies - mindI haveto get to the great age of 67 to come to this realisation.

Hi Bob.

Clearly, running clearances at .008" and .010" can't have done any harm! One wonders what effect on performance setting the clearances to .016" would have, one way or the other? What is the tappet noise like?

I agree about modern bikes providing the real performance fix. As well as my stable of old Brits, I have a 2003900cc Benelli Tornado Tre triple, with a quoted 143bhp; probably 120 at best at the rear wheel;and a 1995 Ducati 900SS, with more modest power; around 80+ bhp; butwhich Isuspect is still capable of seeing off just about any Commando (I will duck now and wait for the cries of protest!), and a bike which I rather hope is on the cusp of attaining classic status.

Yesterday I was at the local'ish Triumph dealership, test riding their675cc Street Triple and Street Triple R demo bikes - very nice.

Regards. Ian

I've had a ride on a 675, great bike, tons of torque for its size as good as anything anyone would need for road use.