Recently I pulled out a spare cylinder I had for my WM20. I actually bought it to rob the decompressor assembly for my project.
The other day I pulled it out and thought to measure the bore. It was 85mm - way too big to account for normal wear.
So, I thought, maybe its an M21 jug? Not knowing anything about M21s, the jug looks pretty much the same as my WM20. Are they different external dimensions? Could I convert my WM20 to 600cc with this (and gain... wait for it... 2 additional horses)?
M21 and M20 have the same (82mm) bore in wartime and post war versions..
The M21 runs a 1/16" bigger inlet port at 1 1/16" with matching carb and the piston has a different gudgeon pin to crown height than the M20...
Strokes for these engines are M20..94mm, M21..112mm
You can run a 'long rod' B33 piston (85mm) in unlinered post war cylinders simply by boring them out...This will give you 635cc when fitted to an M21 bottom end but will not fit an M20 due to the pin to crown height being incorrect.....
Pre war M21s had different bore and stroke dimensions of 100 x 85 but I think I'm correct in saying these dimensions were only found in 36/37 early type engines with the '8 head bolt' cylinder..etc. etc.
Later engines had 10 bolt cylinder heads and virtually no parts are interchangeable between early and later types...Ian