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pvc valves for mk 3 commando


does anyone have experance of fitting a yamaha xs 650 valve between the engine and the oil tank.i would like to reduce the air flow in the tank and stop some small engine leaks  all comments welcome  


I fitted a Yamaha PCV reed valve to my Mk3 but not off an XS650. It was from a 2008 XT125R and described as an air valve. £7.95 delivered off eBay.

It doesn't fit as nicely as an XS650 valve but I made a stainless bracket and mounted it out of sight below the K&N air filter. The outlet on the Mk3 timing side was connected to the valve then on to the normal breather inlet pipe on the oil tank. The breather outlet from the tank tower goes into the rear of the K&N for a closed system which works perfectly.

Be aware that if your engine is not in reasonable condition with good pistons, rings, valve guides and seals etc, excessive blowby may make the PCV less effective



The Xs650 and the Xt125 valves are exhaust gas recycle valves, they are reed non return valves so operate at high revs, so go to eBay and search for Egr valve in motorcycle parts and pick one that suits. I fit them to all my bikes.

In reply to by neill_watkins


thanks for info engine is in very good condition i am planning to vent tank to atmosphere currenty running single carb and air filter with no tank tower and wish to stop oil being pushed down pipe.and under bike.


I use another one, its for fitting in a a straight run of pipe so I fit it in the pipe that runs out the back of the upper timing cover where its also on a MK3.


Same reed valve internals but 180 degree not 90 degree.



The XS650 one is a user mod, they never came with one out of the factory. Mike's XS650 site buys an EGR valve and calls it a breather and not always the same one, must be based on price or availability.


ordered the sx one from a site in germany that does after market parts. didnt have much joy with mikes site


The German site is a reseller of Mike's US site, Mike's US site used to work from UK but added hurdles so you go to the German site.


I'm told BMW ones work well. Make a little housing to fit inline to the breather pipe.


But it looks like the Yamaha one is cheaper.


A reed valve, when connected directly to the crankcase works well at high revs as it can resonate - providing it’s not pumping oil.


The ability to resonate is lost if you use an inline valve, as the rubber hose acts to absorb the pulse from the pistons.

You also run in to the same issue using the breather tube from the timing chest on the 850, as the air has to pass through relatively small holes into the timing chest, which means you lose the pulse effect.


You don’t say where you are planning to mount your breather, but if you are using the standard 850 breather tube on the back of the timing side ‘magneto ear’ you may as well just vent to air as standard rather than via a valve.

You will get all the same benefits of relieving crankcase pressure, and easing some of the ‘weak spots’ that lead to potential oil leaks (problem areas include tacho drive, primary side crankshaft oil seal, timing side crankshaft oil seal and points oil seal)


The only way to make a reed valve work in this scenario is to remove the bottleneck between timing side and crank side by enlarging the holes between the two areas, or drilling some more.


Your alternative would be to fit a reed breather on the crank side instead, and block up the breather tube on the back of the timing side ‘magneto ear’ (it is screwed in, and is a 1/2” UNF thread, so is easy to block)

We have done this on our MK3, and it is very effective indeed.



Suggesting the breather on a Mk3 is best left open rather than fitting a reed valve because of oil hose affecting pulsing sounds like too much overthinking taking precedence over what actually works. You certainly won,t get the same benefits by simply venting to air.

Likewise saying the only way to make one work is by drilling extra holes in the timing side crankcase is incorrect and the mind boggles at this unnecessary modification on a healthy Mk3 although earlier models may be different. I initially fitted a reed valve via the tach drive which is directly in the crank cavity. Worked exactly the same as the timing cover position with the disadvantage of untidy hose routing so back to the timing cover it went.

A good reed valve is a cheap and effective way to help minimise crankcase pressure. Endless experiments and volumous words by forum members in the past have touched on this although most people these days are quite happy to fit one as a DIY and move on. 

Most of the valves mentioned here and also other types including the old Motomite check valve used by myself and others can be easily blown and sucked to check operation. Remove the oil tank cap with the engine running you can hear the valve happily working away and feel the one way flow from the breather working.  

It is a big stretch of the imagination to think the pulses will be absorbed by a proper spec oil hose in a meaningful way by fitting a reed valve. If a clear plastic or other type of unsuitable lightweight tubing that flexes is used that is the fault of the installer.

Theoretically the valve should be mounted close to the Mk3 crankcase outlet like the Jim Schmidt Motorsport design. In practice however, using a good quality oil hose and typical Yamaha reed valve it doesn,t really matter. My own setup has a 250mm long oil hose from the crankcase to the reed valve which is shorter than the CNW installation mentioned by Gordon. Both work perfectly and I can,t recall CNW having to deal with hose pulsing problems over the many years their product has been available.

I would say Donald can quite happily experiment with reed valves on his Mk3 by placing them inline anywhere between the timing cover and oil tank as many have done with complete success.


I fitted a Mikes XS650 pcv to mk1 850. It's under the seat, alongside the oil tank. It exausts into the oil tank, which I then vent into a small catch bottle behind the LH side panel. The breather that previously went to the airfilter plate is blocked off.

It has totally cured all the weeps from my engine, and would probably stop leaks from the tacho and 3rd fin up on the head as well (if you have those problems), as the crankcase is slightly negative pressure.

Before I fitted it, after an oil change the oil would go black after 100 miles, now it stays clean for a thousand miles.

The engine power / throttle response is also slightly better at low rpm


I am not saying a reed valve doesn’t work at all. At low rpm, you will hear it happily chirping away. And as you slowly kick the engine over before starting, you will hear the familiar farting sound it makes.

What I am saying is that I feel that a reed valve has no benefit on a MK3 when inserted inline on the rubber hose versus the factory standard setup where it openly vents back into the oil tank.

Think about what you are doing by adding a valve there - the valve opens when the pistons come down - so exactly the same behaviour as the factory standard setup venting to the oil tank in terms of crankcase pressure. Yes, you are creating a slight vacuum when the pistons go up on the compression stroke, but we are talking about relieving crankcase pressure to negate leaks and stop seals from blowing out - and adding a reed valve inline in the rubber breather hose on the MK3 won’t do anything to aid this over and above the factory standard setup on the MK3.

With the restrictions between timing side and crank side plus the damping effect of the rubber hose on the pulse effect, it simply becomes a one way valve that won’t resonate at fast cruising speeds. Plus the further away from the hot engine you mount the valve, the higher the likelihood is that it will become mayonnaisey on the inside and start sticking - again, stopping it from being able to resonate at higher rpm. 

It is, however, a great placebo upgrade, and there is a lot to be said for that.

I have a Combat engine where holes have been opened up to equalise pressure between timing side and crank side (a standard and well documented Norton factory note from back in the day), in lieu of the Combat froth tower I have fitted a reed on the ‘magneto ear’ it is actually one of Jim Schmidt‘s valves - I think the reeds he uses are BMW. It works ok.

We also have one of Jim Comstock’s sump plug reed breather valves on the MK3 and have blocked up the standard breather tube location. And that works significantly better than the Combat.

Taking it to the next level, we have now rebuilt two 650ss engines, where as well as removing the timed breather, we have blocked all the air pathways between timing side and crank side except for one 1/4” oil drain hole. One bike has a reed valve on the back of the crankcase (a Comstock/cNw unit in the Combat location) which works very well indeed. The other has one of Jim Comstock’s sump plug reed breather valves, and this is by far the most effective of all of them.

I have spent a lot of time and money studying and testing various solutions (on other bikes too) including diving heavily into the research into engine breathing and the effects of blowby which was done by Rex Bunn. It’s a fascinating subject that I am quite passionate about, and would certainly recommend that people experiment, as there is a lot you can do to improve things!

Don’t confuse Holland Norton Works with Colorado Norton Works - they are two separate companies linked only by the fact that Constant Trossèl (also a very knowledgeable Commando guy) at Holland Norton Works sells some Colorado Norton Works parts. The breather sold by Colorado Norton Works is attached to the back of the crankcase, and was designed and manufactured by Jim Comstock - who I would say is one of the best and most knowledgeable Norton engineers alive.

Jim Comstock’s sump plug reed breather valve is now manufactured and sold exclusively by Kenny Cummings at NYC Norton.

I don’t believe I am ‘overthinking’ things and similarly I don’t believe that anything I have shared is ‘not correct’ 

Engine breathing is one of those emotive topics that will be discussed forever more. There are lots of differing theories and opinions on this. All I can do is share mine based on my research and experience with several of the solutions available in the market.


Resonate frequency of reed valves is covered by a calculation where the parameters are


  1. Reed Petal Material
  2. Thickness
  3. Free Length



Whether an owner chooses to fit his own simple but effective setup using a Yamaha type reed valve in the original oil line or a Schmidt/Comstock/Cummings device is up to him. The US parts are very nice indeed but in the case of the Comstock/Cummings valves, around £300 delivered to the UK.  Jim Schmidt's one is cheaper at £200 or so and he offers versions that fit directly into the Mk3 threaded port or inline. As far as I know he doesn't say one is better than the other or that the crankcases need drilling.

He asked me a while back when I enquired on another matter if I,d like to try the original type valve which fitted the Mk3. He wanted to know if it would foul the Prestolite starter. I told him I was happy with my Yamaha setup but thought it could possibly. His subsequent tests resulted in the inline version for Prestolite Mk3 fitment which would be my choice anyway as it is a far neater installation. With these US devices or a DIY approach there is no need to overthink, all the necessary R&D has been done by others, make your choice and move on.

Spending a lot of time, money and reading on the subject doesn,t make your approach superior to a simple inexpensive mod that obviously works. To smugly dismiss these efforts as a placebo effect could just as easily apply to your own "significantly better" improvements with the added incentive of having to justify the time, money or extensive research.

My setup works fine all year round with no oil leaks but current mileage is reduced to around 5000 miles per annum due to no touring Covid. Mayonnaise oil is not a problem if the engine is properly warmed up. A minus 1c start to a ride earlier this week including normal motorway speeds and some redline moments produced no such effect at the end of the 100 mile run.  I would add there is also no oil smell around the bike if the oil tank breather outlet is routed to the air filter. My early open pipe on the back mudguard usually resulted in an occasional oily whiff..


Really happy to agree to disagree with you Neil - as I said previously, this is an emotive subject with lots of differing opinion.

I was merely sharing mine, just as you have shared yours.


I wasn't aware I was being smug in any way, if you took it like that, I apologise.

I was merely putting forward my viewpoints based on my engineering understanding and the simple fact that putting an inline reed valve on the MK3 breather pipe simply cannot improve the issue of crankcase pressure blowing seals and leading to leaks how can it when nothing else has been changed.


And with a display of hostility like yours, I am now out on the 'discussion'.

thanks for all your comments will try the xs 650 valve first i am not expecting it be delvered  till the end to next week. i like the  catch bottle behind the panel .it,s not a big problem so i don,t wont to spend 300 pounds fixing it but am greatfull for the info .


I have been reading this thread with a mixture of interest and bemusement, wondering why, on s Mk3 at least, a crankcase breather valve seems to now be considered necessary?  My first Mk3 was a 1975 Interpol that I acquired in late 1979 via a sales outlet straight from Norfolk Constabulary; consequently, its mechanical side was in completely standard condition.  After sorting the bike out cosmetically (it was a bit unloved even after only 4 years), it was then my sole transport for 3 years until an accident (not my fault) badly damaged it.  In that time, I travelled most weekends between RAF bases to home in London, round trips of 160 and 220 miles, mainly on fast A roads and motorways, typically  cruising at 80+ mph.  During the week and weekends, the bike would also be used for local trips, so I would generally be doing about 250-300 miles a week, 12 months of the year.  

In all that time, apart from a phase of smoking that was cured by a rebore, the only recurring issue I had was a slipping clutch.  The engine did not leak a drop of oil, not even staining anywhere, which was a revelation after my previous 1968 Bonneville.  It did not use much oil either and I had never heard of wet-sumping until I started reading Roadholder.

As a result, you may understand why I am struggling to see why any alteration to the standard breathing system would be needed - it worked for me, and my present Mk3, which is currently in the assembly stage after being bought of eBay as a complete wreck, will again have a completely standard engine and breathing wise.  I am not after ultimate performance, just reliability for longish trips, as before.  The first Mk3 was reliable and fault free in standard condition, so I see no reason why this one should not be too, without any modification apart from Rita ignition.  That change is to eliminate points maintenance, though the points set-up worked perfectly well for the first 2 years until I converted to the Rita purely for the aforementioned reason.

So given my experience, to my mind, a breather valve seems unnecessary.



There was an article back in 2013 which actually has some measurements on what a one way valve did to crankcase pressure. Given all that has been written above it may be worth having another read. I am a great believer in 'numbers' to remove any possibility of the quite valid suggestion of a placebo effect.

Roadholder 302 January 2013 page 22




Well despite all that has been said above I have fitted a yam 125 valve in my crankcase breather and it has made a remarkable difference. I no longer have to wipe a film of oil off the head and side panels after a ride, when I regularly take it up to 5000rpm. Just saying....


 fitted the sx valve today have fitted up by the oil tank and it makes for a out of site installation .cleaned the engine first .on startup air reducted to zero within a short. space of time have been for a 60 mile ride and no sign of the small oil leaks will put some more miles on when weather improves.then test the yamaha 125 one i have and see which one i like best thanks for all help and comments regards don


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