flap valve in crankshaft allowing oil into chain case

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Re: flap valve in crankshaft allowing oil into chain case

Postby Groily » Sat Sep 19, 2015 1:19 pm

Well . . . yup, it is quite small, but mine has lasted several years and many thousands of miles on my very similar-to-yours-500. After machining it down I did heat it to cherry and dunk it in oil to (at least partly) harden it so the seal lip wouldn't wear it away too fast. If everything is done up properly, don't see why it should get knocked about too much, but obviously there is some squeeze applied.
I think - not quite sure - that the original articles written on this were aimed at 650s with no shock absorber though ('cos they have cush drive AMC clutches etc), but I thought what the heck, and just went ahead, even if it's a bit more delicate to look at on the older smaller engines. Works fine, but can't promise that in another few years I might not have to make another ring! It certainly hasn't been dashed to pieces

As for computers, I hate em and have literally no idea why/how they work ('Saltbox Alf' sorts ours out every time he passes by, poor sod, because he does know!) But depending on where you are and stuff, you probably get different results from the same searches.

Good luck with the assembly. Probably need a bit of heat on the crankcase to persuade the seal to push in as I think I said somewhere - I know I did. I made a little mandrel to get her in square. But she went in, and oil does not pass.

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Re: flap valve in crankshaft allowing oil into chain case

Postby bjork » Sat Sep 19, 2015 6:46 pm

The last bike I did this mod to was a 600 with crank shock absorber. That worked fine for me for thousands of miles including trips to Switzerland etc and the new owner has had no trouble either. So if there is not enough room to leave a flange on the flinger as I've done with my 650, then go ahead with the fully machined job.
Ref your breather troubles, I cant help feeling that the pipes open to atmosphere leave the valve with nothing to do. So would think maybe it would be better to route all your pipes into one container and fit that with a one-way valve to atmosphere?
While those pipes are still open they will draw in air on the upward stroke so starting the pressure cycle all over again.
Keep at it though, you'll fix it!
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Re: flap valve in crankshaft allowing oil into chain case

Postby rump » Sun Sep 20, 2015 8:52 am

Groil,Do you quench in vegetable oil?I will start next week softening an old ball bearing centre and machine it down. bjork. If put pcv valves in line i dont think the light weight plastic balls could handle the change of direction when you think of the revs per second of the air pulses. My theory is there is a positive presure getting the air out where as using vacuum to fill the sump would not or should not be better than 50 % efficent that is my theory this week.

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Re: flap valve in crankshaft allowing oil into chain case

Postby bjork » Mon Sep 21, 2015 6:37 pm

rump wrote:Groil,Do you quench in vegetable oil?I will start next week softening an old ball bearing centre and machine it down. bjork. If put pcv valves in line i dont think the light weight plastic balls could handle the change of direction when you think of the revs per second of the air pulses. My theory is there is a positive presure getting the air out where as using vacuum to fill the sump would not or should not be better than 50 % efficent that is my theory this week.


If you have open pipes, crankcase pressure will find the easiest way out which is the open pipes. So the only valve you now have is doing nothing. When the pistons rise the vacuum they create will draw more air in, combine it with combustion blow-by on the next down stroke and create a greater pressure than it had before. That's when you need valves, to shut before the pistons rise again, leaving the cases with a vacuum and only the blow-by to deal with on the next down stroke. So less pressure to pass the valve than before and reduced oil leaks too. I don't think the valve works as often or as hard as we might at first think, assuming it is actually working and sealing effectively. So those light weight valves might well do the trick.
If you have multiple valves, only the ones that open at the lowest pressure will lift and if they can pass enough gas that is all the engine will need.
I still think my theory is better! ;) Honest Guv, valves are the way to go. If your standard crank valve is working and the engine isn't worn out you won't need anything else. My model 31 AJS has it's nuts revved off it with it's low sidecar gearing, and that doesn't have a breathing problem either.
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Re: flap valve in crankshaft allowing oil into chain case

Postby rump » Tue Sep 22, 2015 5:53 am

Hi there I have just finishing making new spacer to fit the 52x40x7 seal will harden it tomorrow .The breather pipes run to a block of aluminium with individual inlets which will take the valves I made it that way in case they are needed. I just found another miss alinement with engine sprocket and clutch sprocket by one eighth of an inch. rump.

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Re: flap valve in crankshaft allowing oil into chain case

Postby bjork » Tue Sep 22, 2015 12:19 pm

rump wrote:Hi there I have just finishing making new spacer to fit the 52x40x7 seal will harden it tomorrow .The breather pipes run to a block of aluminium with individual inlets which will take the valves I made it that way in case they are needed. I just found another miss alinement with engine sprocket and clutch sprocket by one eighth of an inch. rump.


That mis-alignment will need looking at for sure. Ref the valves again, if you have the pipes into one block then a single outlet valve there will suffice. If you still have trouble then try blanking off the original while testing how well the new valves are doing.
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Re: flap valve in crankshaft allowing oil into chain case

Postby Groily » Sun Sep 27, 2015 8:27 pm

I just use engine oil rump, whatever is lying about. Although the quenching routine does a reasonable job, it's always going to be a bit hit and miss compared to 'proper' heat treatment I reckon.

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Re: flap valve in crankshaft allowing oil into chain case

Postby rump » Fri Oct 02, 2015 2:05 am

Hi, Groily, Dad has fitted the seal and made a new spacer from a KTM main bearing roller race inner and hardened in salt water an old trick Ran the motor with chain case off no oil leak, with all the breather pipes open to space the one in the crank shaft was not breathing so now to start fitting none return valves one at a time only trouble is dad has just had a stroke so everything is on hold. Rumps son.

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Re: flap valve in crankshaft allowing oil into chain case

Postby Groily » Fri Oct 02, 2015 6:34 am

Very sorry indeed to hear about your dad's stroke - best wishes to him for a good recovery. There's no justice . . .
Well done with the seal though - and per what Bjork and I both said somewhere said I think, you probably won't need the flapper valve, what with the other breathers - or you might find you can dispense with the auxiliary plumbing after all and keep it standard.
On one of mine, I have a large additional breather coming off the timing cover, about a 1/2 or 5/8 inch id tube off an angled fitting attached to the cover, which was lightly milled to get a flat surface. That runs in conjunction with the standard valve in the crank (on a later 650 engine). Very effective.

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Re: flap valve in crankshaft allowing oil into chain case

Postby gtx469 » Sun Jun 21, 2020 5:36 pm

I see a lot of posts saying that there is a spring in the flapper valve crank nut breather. I just did the oil seal mod to my 61 G9 and my crank breather nut does not have a spring in it, just a small stainless steel plate held in by a peened edge ring. seems to work just fine blowing air out to case but seals going in. Is this nut something that was changed in 61 era?
Larry Larivee

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