Are there any differences in these barrels and can I use an M21 barrel on my WM20?

email (option): s_lord@talktalk.net

Steve. The bore size is the same for both M20 and M21. The same barrel is used. Although the inlet port could be slightly larger by 1/16" on the M21. It's the stroke that is different... plus a different configuration of the piston. Ron

email (option): ronpier@talk21.com

It are the flywheels and the piston that make the difference between an M20 and M21. The M21 also has a slightly bigger carburettor.

Henk.

email (option): ahum@quicknet.nl

Later barrels also have a thicker base flange...and longer base studs to suit so depending on the year of the barrel you may need to change the studs. The shorter studs will work but won't extend the full length of the barrel base nuts when fitted....Ian

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

I didn't knew that

email (option): ahum@quicknet.nl

Henk..It would be no fun at all if we all knew everything about everything..you're always telling me things I didn't know about the early bikes! (just goes to show I'm not really an expert)...Ian

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

Thanks chaps, still a bit confused, so is that a yes? Could I drop fit this barrel onto my M20?

email (option): s_lord@talktalk.net

Sorry if I was instrumental in confusing you! But Yes the M20 and M21 barrels are the same. Ron

email (option): ronpier@talk21.com

I think the vey early M21 barrels have a different bore size (or am I mistaken?). It's worth mentioning that. I doubt there are many early ones around, if any. But I would certainly measure the bore just to make sure (or ask the seller to measure it). Someone might be trying to offload an old pre-war M21 barrel that's not of much use to most people.

As I understand it, M20s have an 82mm (bore) by 94mm (stroke). M21s (post 1938 or '39) have a 112mm stroke. But pre-1938 M21s have an 85mm bore (by 105mm stroke). One of you experts out there will correct me if this is wrong, I'm sure.

email (option): dannydefazio@sumpmagazine.com

Your right Danny..early pre war barrels also have no air gap behind the tappet chest, 8 headbolt holes instead of 10 and a slightly different fin pattern (no 'heat break' in the fins on the tappet side.) I think they also have 4 not five base studs..can't quite remember!
...Ian

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