The breather mechanism on my 49 M20 is located in the timing cover. If the bike is left idling for a minute or two after a ride I have noticed an oil mist stain on the pavement from the breather as well as droplets of oil. While not excessive is there a set amount which is acceptable or should there be no oil escaping from the breather? Thanks in advance.

There will always be some oil mist, which on earlier models anyway, was directed to the rear chain, Excessive amounts could be related to bore or piston ring wear. Ron

email (option): ronpier@talk21.com

Thera are different designs to the timing cover as below. The lower one limits the amount of oil splashed onto the breather to some extent. Orientating the breather hole towards the cover and away from the pinions can also help reduce the amount of oil getting into the mechanism

The piece that retains the flap disc within the breather body was also altered three times....Along with the multiple locations of the breather over the years on B and M Series models and in the case of the B30, four breathers fitted to one engine type indicates BSA had ongoing probems with this..In all cases the later parts are the ones to go for..So, squashed down and cross drilled tube in the breather body, the later timing cover with the 'shroud' cast into it and the last alloy flap valve retainer...However, I'd say that oil loss should only be minimal and not particularly noticeable, so check the valve is working and sealing properly and that you do not have an underlying problem that is increasing crankcase pressure...

It is also noteworthy that on freshly rebuilt engines some oil will 'spit out' from the brather until the rings have bedded in..I have tested this on a number of engines and concluded it lasts for about 150-200 miles and then stops...Ian