Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
Author
Comment
M20 vs M21 cylinder

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)?

email (option): cas.vanderwoude@gmail.com

Re: M20 vs M21 cylinder

The difference is in the stroke not the bore.

Re: M20 vs M21 cylinder

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

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

Re: M20 vs M21 cylinder

hi Ian,
The previous owner may well have done that as the jug came with a B33 piston which is a snug fit

email (option): cas.vanderwoude@gmail.com

Nieuwe pagina 1