Would it an advantage to use a timing chest cover with a breather 
John 
email (option): john.kater@chello.nl
Not really John...The M20 doesn't suffer with breathing problems at all with the wartime set up..Even at the increased cylinder capacity of the M21 (600cc) or in my case, 720cc, the breather copes quite happily with the extra displacement...
BSA moved the breather around a number of times and the breather itself went through a number of variations, with detail changes to the flap valve retainer and the outlet pipe diameter. Also, on the timing cover type there were changes to both the cover and the pipe that protrudes from the breather assembly inside the cover..
Changes weren't necessarily made to suit the M Series models...Changes to help the B Series engines would have been standardised just to make production and spare parts supply more easy...
When I 'bumped' my B33 to 600cc the breather (in the cover) was fine, but at 720cc it couldn't cope and I had to add an extra breather..The B33 however, revs higher than the M20 and that combined with the capacity increase was what led to the problem in that case...Ian
email (option): ian@wright52.plus.com
It's worth mentioning in the context of Trevors comments that the flap valves tendency to not know whether it's 'coming or going' at higher revs led to the introduction of the 'timed' breather on the Gold Stars to keep everything operating as intended....
As I said I've never had problems with the breathers ability to move the air on the sidevalves..only on OHV models...
When sprinting a B33 for example which was making peak power at 6800rpm and with a max rev limit of 7250rpm the standard breather struggled but was OK once a larger diameter second vent pipe was added (also with a one way valve fitted)...
I'm assuming in making these comments that any engine under discussion is within the limits of mechanical tolerances for the type....Ian
email (option): ian@wright52.plus.com
By the time you get to 5500 rpm it starts working backwards.
With the big carb on, mine would do 6000 and at those speeds it pumps oil out the breather as it is totally out of sinc with the engine opening on the upstroke and closing on the downstroke.
Max rpm of a M20 isn 4200.
John
email (option): john.kater@chello.nl
Thank Ian,
Because oil is coming out of the breather I replace that with the one in the timing chest cover. It would be an update
John
email (option): john.kater@chello.nl
My breather has no membrane,
So it works both way, and few other bikes I know are the same.
Does it really matter? If so, how?
Is it better to have a kind of vacuum in the cases?
In bigger 4T motors, such as cars, the breather is just a vent.
Thanks,
Noam.
On Std engines without oil seals on both ends of the crank the idea was to run negative pressure in the crank.
The logic behind this is when running air gets sucked into the engine past the slingers and scrolls and draws the oil back into the engine with it.
On engine with proper seals there is no advantage in running a crank vacuum.
When Rex was testing the breathers the Uni od Aukland found some engines yielded up to 2 Hp more if air was allowed to enter & leave the engine with minimal restrictions.
low pressure under the rings actually hampers their operation and aids fuel being drawn into the crankcase.
Pumping air requires energy and generates heat and neither of these are beneficial to engine performance.
In theory running an inlet and exhaust breather on a side valve with a sealed bottom end will yield a small power increase.
The problem with doing this on an M20 is
1) crank is not sealed
2) existing breather holes are too small to allow air to be sucked in without drawing a vacuum in the crank and to allow air to be blown out without compressing it
So on a high reving race bike ( are you listening Drew
) you can get some more horses by fitting an inlet in the crank and an outlet in the timing cover but they need to be 1/2" to 3/4" and I think you will find a sutable on on the crank breather on a Ymmie SR 500
email (option): bsansw1@tpg.com.au