crankcase breather - where from in UK

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Oct 16, 2006

My '72 Commando has it's breather at the back of the engine, and I want to fit a one way valve into the vent pipe.

Does anybody have a part number for a suitable item which is available in the UK? The references I have found in the archive are for parts which are available in the USA - and it seems they are not available in the UK.


What about the Norvil item..........
069903 VALVE - BREATHER PIPE - 1/2" - 200000 ONWARD & ALL 850 & 920 C - - - 21.45.
Might only work successfully if your sump does not fill with excess oil when running at 4000 +. Original breather carried oil back to tank as well as air according to the mythology! I never could see how with the foam filters in place on Combat breather but I guess it was possible.
The commando 1972/3 750 models have an inherent manufacturing fault with the breather and oil returning to the tank.

The breather on the pre 1972 models was on the left hand side of the crancases by the barrels. The problem arose when it was moved to the rear at the bottom of the crancases. The oil mostly returns through the breather and not the oil return pipe.

To cure the problems on my 1972, A plate was inserted into the crancases to stop the oil being pushed away by the pistons allowing the oil to pool and scavanged back through the oil return pipe.

I moved the breather up to the top of the timing case where the magneto used to be and bolted a plate onto the old breather hole.

The oil system is now working fine on my engine.

Here is an article on the subject
I should have mentioned that mod which I did it on my original Combat cases. The oil migration to the rear is so bad that the front pickup is can be uncovered at medium to high engine speeds. Mine would empty the tank in 10 miles at 70 mph and I think when the engine bearings were new the problem is hidden partly because of the breather moving oil back to the tank and because the rod bearing clearances were tight enough to minimise the flow. Mine weren't knocking but were twice the clearance, enough to make the sumping problem worse. The PO had moved the breather to the timing case making it impossible for oil to return at high speed. Definitely worth the effort in stripping the engine.
Thanks for the info, I'll try the Norvil part.

My reason for installing this part is to try to stop an oil leak from behind the crankseal (the one behind the alternator). I managed to stop a variety of leaks from other places, and once I had done so the leak appeared from the crankseal (which had not leaked originally).

I replaced the seal, loctited it into place, but after a a hard run it still weeps, so I figure it is because of crank case pressure build up. The breather pipe runs into a small catch tank - not much oil ends up there. I have not had the cases apart in the five years of my ownership, so I don't know if the scavenging is standard or modified.

Reading the archive, these valves seem to cure everything so it's worth a try.

Thanks again,

Keith1069 said:
What about the Norvil item..........
069903 VALVE - BREATHER PIPE - 1/2" - 200000 ONWARD & ALL 850 & 920 C - - - 21.45.

Hmmm.... looks like something you might find at a plumber's supply. Anyone have one and can enlighten us on what is inside? Is it a reed valve? Check ball?
Don't be too hard on Mr. Emery, he had to add two hose barbs to it! :roll:
crankcase breather

The one I installed I found at O'Reilly's auto parts and is a power brake booster valve, fits a 1/2 id hose on one end, the other turns 90 degrees and is 5/16 or so which works great for a stock connection on an 850, but nothing I own is stock. Cost about three bucks.
I am using those also, but most modern high performance stuff use reed valves. Supposedly the rubber flap in the brake booster cannot react quickly enough to be effective at high rpm. Same could be said for Les Emery's spring-type check valve. There is a BMW unit that I have seen modified for Nortons and I have been thinking about the S&S design for high performance twin cam H-D's .

But for now the brake booster valves keep the engine clean. Not enough time to get to everything.
Interesting......that's where they come from then. Mine works well but does need cleaning every year as it fills with mayo although performance does not seem to be affected. So, with two 1/2" pipe fittings price is about the same as Norvil but worth a look if ever the valve needs replacing.
Did anyone see he's trying to sell a Drouin blower kit on Ebay? Guess whoever buys it will be coming back for new cases, crank and rods!!
I have bought one of the Norvil (1/2 lb) type one way breathers, and cannot find space to fit it. I have an 850 mk3 with the starter in situ and an earlier type air filter on. On my bike, the breather goes up behind the right side of the air filter housing and straight to the tank.

Although it may just about fit (at a funny angle) behind the air filter, with it being so heavy and made of brass, I'm scared of it damaging the machine. Also there will be quite a sharp angle on the pipe as it leaves the valve.

The only solution that I can see is to route the breather pipe under the air filter body to the left side of the machine before it goes up behind the air filter and then fit it behind the air filter, and even then it won't be easy. It's 6" long with the hose barbs fitted! I suppose I could take 1/2 inch off each barb.

Has anybody else managed to fit one to a MK3 successfully?
I fitted a Guzzi breather valve, a simple retained lose steel ball. I did use a VW valve for a couple of years but it would block with mayo.
The Norvil valve was and perhaps is a standard conex check valve and in my opinion takes too much pressure to relieve. It might work better with the spring removed, kept vertical and the right way up to let gravity close it.

The Guzzi valve is pushed inside the breather pipe, it's a bit too big for the Norton pipe, but hey! I'm sure I could make a couple of modified versions. Anyone interested?

I fitted a Guzzi breather valve, a simple retained lose steel ball.

Isn't this the way most American car PCV valves work? It seems that you could find one of these that would plug into the hoses easily. Many are quite small and should be easy to fit.

I am told these do not work as efficiently as the power brake booster valve or reed valves.
old grey biker said:

My '72 Commando has it's breather at the back of the engine, and I want to fit a one way valve into the vent pipe.
Does anybody have a part number for a suitable item which is available in the UK?
Thanks Alex
Any VW dealership will help with a brake booster valve for a Golf/Jetta/Rabbit - if you need a new one.
If you want to go the cheap route to experiment, pull one off a VW car at the local breakers - the breaker for older cars that you pick the parts - pull out the valve and cut off the hose - simple 5-10seconds. When I ask for a price they shoo me out the door. The valves are like new, even in a 15 year old car.

Did you drill holes in the case, per
It would seem the crankcase would breath through the main bearing on the timing side. I noticed a thru bolt holding the magneto blanking cover. Did the threads strip out? I had that happen. I realize it's tapped from the back side, so you can't use a regular Helicoil©. There is a backside Helicoil© insert one can use.
The guy who did the engine for me was Colin of Thor motorcycle broken link removed He has 3 commandos and just loves the bikes, he will build one to your specifications.

He inserted a plate to stop the oil cavitating and make it pool so it could be returned through the scavange hole to the tank, blanked off the breather hole and moved the breather to the rear of the timing caese where the magneto would have been.

No valve needed on the 750.

The bike runs fine at all revs returns oils as it should - problem solvedFor those without the modification it is a problem waiting to happen
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so, I went to a local car breaker yard, and had a look.

A volvo 240 servo seems to have the right sort of valve, the end which pushes into the servo is the right size for the pipe coming off the crankcase, and the other outlet also seems to be the right size. It certainly is one way as you can blow through it in one direction but not the other. I also picked up a valve off a Rover 600 and a Citroen, the sizes are not as perfect but if the volvo bit does not work I can try make them fit.

Took about two minutes to fit the volvo item.

Weather permitting I'll run the old girl over the week-end and let you know if it works.
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