An interesting read on the subject is contained on page 397 of the book 'Tuning BL's A-Series Engine' by a Mr Vizard..
I understand that a few years ago BMW did a bit of serious research into crank case breathing and came up with a reed valve. It is about the size of a 50 pence piece but a bit thicker. The last one cost me about £7 but that was a long time ago.
I( modified the 'Manx' alloy oil tank of the el cheapo so called 'classic' race bike so that it sat on the top venting to atmosphere and ran a breather pipe back to the tank from the rear top of the timing chest about 1/2 inch OD if my memory is correct. When the engine was fired up the reed could be heard clicking away for the first few rpm but quickly stopped and from then on would only do so very occasionally probably due to blow bye on the rings building up a bit of pressure in the crank cases.
I would never build a bike without employing the system IF I were ever to get my proverbial finger out and do so.
A breather system used I believe by Triumph at one time and tested by Mr Oswald (Quiet Power Drives)for use with his belt systems is to run the breather back to the oil tank and to put a tiny hole into the filler cap. When the motor is first fired up as the pistons come down crank case pressure pressurises the tank..some air is vented via the hole to atmosphere but when the pistons go back up again the tiny hole allows very little back into the tank and after a few revs everything settles down to just about atmospheric pressure.
Personally I never understood the late Triumph idea of breathing into the chain case and was told by a Service Manager that it was only done to ensure correct oil level was maintained in the chain case as they had found many owners were incapable of doing so and bikes were coming in for service with rusty damn nigh solid primary chains...Of course the 'brains' at Umberglade Hall removed the B50 crank oil seal to allow the motor to have two breathers(!!) but forgot to include the 3 little drain holes to allow excess oil to drain back into the crank cases which results in oil flooding the dry clutch resulting in slip and dry problems...all very clever I do not think!!
