Hello Gents, I have a ZM21 and a ZB33, both motors are currently on the bench for rebuilding, I have two quite different timing chest covers, with two very different provisions for crankcase breathers. I'm wondering if anyone can shed any light on the evolution of these covers, and what the most appropriate breather setup for either, or both, of my motors might perhaps be?
Regards,
Nick

The cover on the left is the same as the ones fitted to our WM20 engines where the breather is on the drive side crankcase. The one on the right with the flying 'B' is post war when I think they changed the breather position. I'm sure Ian can give the change over date.

You have an odd screw where the ball and spring sits on your first cover. Ron

email (option): ronpier@talk21.com

Hi Ron,
the one on the left is sligtly different from most m20's cause it doesnt have the extra "flesh" where the breather would come later.
It is a bit rare, therefor.
I think both engines should have the one on the right.

the one in the left for wm & xm & ym 20 - the another one for zm & bm 21
but all is one size
cheers
hussein

email (option): sl_hussein@yahoo.com

Thanks Hussein for clearing that up!

Nick it's something else I'm not sure about. My own WM20 with early cover doesn't have the extra "flesh" and nor does the later cover on the 1942 RAF outfit I have here? Ron

email (option): ronpier@talk21.com

Have you a clearer view of the Numbers on the back of the castings.

are they both 66-1919
I see one seems to be 66-1919-BSA

Both cover are actually stamped 66-1919.

In regards to the breather assemblies, if I plug the breather with just a plain bolt and instead run a hose from the tappet instecption cover (B33) at the bottom of the pushrod tunnel would that be just as effective? I'll be racing the B33.

If you are going to race the engine you would do better to fit a Bunn if you can find one.
Run a through system, in through the out door ( oh oh I going all 70;s now and dancing around in a caftan ) and out through the rocker cover.
One way valve in each.
When Auckland Uni did the testing they got up to 2 Hp improvement due to energy not being used to compress the gas in the crankcase and maintain the crankcase vacuum.

If you use clear tubing on the exhaust you can watch it running under UV light ( forget the mirror ball, now I am going 80's ) you will see the nodes.
mark them at whatever you peak power revs are and cut the tube that length to get maximum flow, not much different to tuning an exhaust pipe.

email (option): bsansw1@tpg.com.au