The outer drive side main bearing of the M20/21 (and B Series engines) has a steel shield fitted into the crankcase behind it to limit/prevent oil getting into the primary chaincase.
This does not form an airtight seal so it follows that the engine will 'breath' through it to a degree when the piston is on the downward stroke.
In effect, therefore, it is likely to be supplementing the action of the crankcase breather.
I have discarded the tin shield in my 720 B33 engine and replaced the standard bearing with a sealed type, removing the inner seal to ensure good lubrication of the bearing.
I am wondering if this has any effect on the pressure in the crankcase. If it increases crankcase pressure by the removal of the ability to 'breath' at this point, will it increase the likelyhood of oil leaks?
Despite careful assembly my B33 leaked oil more than I would have expected. Of course with such a large piston there is more air to be displaced and it might be considered that the breather has gone past the limit of its capacity. However, I have not had a similar problem with my M20 which is of the same capacity but has a standard bearing and shield fitted.
Has anyone fitted this type of rubber sealed bearing and if so have there been any oil leak problems?..beyond the normal amount!...Ian

email (option): ian@wright52.plus.com

maybe the crank pressure could have blown the seal out on the bearing

Although the engine may breath slightly through the bearing, the breather should be big enough to cope. If you've increased the cc the breather may need enlarging to suit. As the engine is acting as a large set of bellows it might be worth putting a breather valve in line on the breather pipe. This allows air to blow out the breather but closes on the up stroke creating a slight vacuum. The lack of air pressure in the crankcases reduces the chance of blowing gaskets and oil leaks. There are various value available, some use a rubber disc for the valve and for higher revving engine there's a reed valve type. I would have thought the rubber one would be fine for a single. There are lots of websites and discussions about the subject. Here's a link to a brit bike forum about it.

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=70933

email (option): horror@blueyonder.co.uk

If one was to fit a vacuum type breather to the crankcase vent, wouldn't it have the opposite effect of sucking oil out of the primary case once negative crankcase pressure was achieved/ Just a thought.
Vincent

email (option): vinver@ns.sympatico.ca

The BSA breather already has a one way flap valve in it. Which will create a partial vacuum on the up stroke. Ron

email (option): ronpier@talk21.com

The interesting thing is that my 720 sidevalve doesn't suffer from the problem despite being the same capacity and having the same breather set up...other than the fact it's location is different.
As Ron says the standard BSA breather has a 'one way valve' built into it as standard.
I think I may just fit a second breather and see what the results are when I run it...Ian

email (option): ian@wright52.plus.com

I remember fitting MASSIVE vacuum pumps to the crank case on the big drag bikes we built in the shop. Somewhat unrelated but the more vacuum you have the less resistance you have in the crankcase.

email (option): micran1234@yahoo.com

Hi Vincent, I guess you're right if any can get past the bearing and the oil level is that high in the primary, but as the valve isn't fitted on the bearing it would blow oil back into the primary on the down stroke. It can't be very much as the breather will be the point of least resistance if it's big enough for the job.
I never really took much notice of breathers in the past until I pulled the breather pipe off the airfilter on my Harley and just pushed the pipe down the back of the gearbox obviously kinked. This resulted in blowing every gasket on the engine. Now I make sure the engine can breath.

email (option): horror@blueyonder.co.uk

Could it be that the air has a more convoluted route to get the air out? On the M20 it is on the left hand crankcase, and it is an easier route for the air to exit, rather than going through the timing gears etc to exit through the timing cover

email (option): davmax@ntlworld.com

2 beathers one on the crank cases and one of the later models camshaft case i am not sure what help the breather hole in the magneto cog does

Hi Roger..If you are talking about the late Goldie breather they found that as the revs got higher (6500~7000 rpm) the old 'flap valve' breather couldn't operate fast enough and was opening and closing at the incorrect time in relation to the downward stroke of the piston. So, on late Goldies they fitted a mechanically driven timed breather, driven off the mag drive gear, to solve the problem..Ian

email (option): ian@wright52.plus.com