On a WM21 piston, is there a 4th carbon ridge near the top or is it missing. And is it concave on top as well? If you happen to have any photos of one of that period, this would be appreciated but not absolutely necessary.

email (option): unpob@yahoo.com

Further on that piston, found the part # 66-854 which is listed as M22 BSA. Wanted to confirm that this piston was also used on WM21.

email (option): unpob@yahoo.com

Hi Henri...The M21 piston doesn't have the 'empty groove' like the M20 and the top is dished slightly...I'd have to check further to determine whether it is the same in all respects as the M22 piston..Hopefully someone will know...Ian

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

Thanks Ian, this one is dished as well. Here is a better photo and also received my JM21 head today. Getting there

email (option): unpob@yahoo.com

Hi Henri...Nice cylinder head...I've been looking for one of those myself. I had one a while back when I didn't need it and sold it to a guy in China..now I'm regretting it...Ian

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

Hi Henri,

Yes the 82mm M21 piston is also used as low compression piston for OHV 500cc 37-40.
It makes the OHV a bit sluggish, but allows use of low octane fuel, easy starting, sweet running and very unstressed.

The thicker SV rings don't pose a problem.

As I recall, some parts books actually list the M21 piston for OHV use.

Neale

email (option): n.gentner@bigpond.com

Thanks Neale!

email (option): unpob@yahoo.com