What are the visual differences between an M20 and M21 crank? I have a couple of M20 cranks on the bench but the only M21 variety is hidden away in the bowels of the bike! I will be looking for an M21 crank soon and don't want to end up with another M20 one out of ignorance!
Just checking.. An M21 crank will go into M20 cases won't it, or have I missed something???
Cheers
Pete

email (option): petercomley@web.de

Hi Pete..Later cranks used the same forgings and numbers, just being machined differently to suit application. Both types will fit into the same cases and use the same conrods. The easy identifier is the stroke difference..the M20 is 94mm and the M21 is 112mm. So, just measure from the c/line of the mainshaft to the c/line of the crankpin with a rule. M20 will be 47mm and M21 56mm...It's pretty much impossible to get it wrong with a 9mm crank throw difference..Steel flywheels have forging numbers begining with '6'..eg 65-635, Cast Iron wheels begin with '7'...eg..65-734..Go for the steel wheels if you intend to machine much off them, either type if you are building with a standard spec. crank.....Ian

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

Cheers Ian, I knew you would set me straight.

Cheers
Pete

email (option): petercomley@web.de