well well well i need some detail please :

the bore on my machine is 1 1/16 is the engine an m20 or m21 and is the m21 engine a 600?

email (option): david.bonello@go.com.mt

You must be talking about the Carb Bore. I'd say it's virtually impossible to tell the difference between an M20 or M21 from the outside.(Apart from more compression at the kick start) Even the piston diameter and the con-rod are the same. The big difference is in the crankshaft which gives an increased stroke of 112mm on an M21 (591cc) as opposed to the M20's 94mm.
That would be easy to measure with the head off, or even through the timing hole in the top of head with a piece of wire or drinking straw. Ron

email (option): ronpier@talk21.com

cheers

yes it is the carb bore ,

and are the cylinders interchangable ,so could i have an m20 with the cylinder of an m21?
thanks

The cylinder are the same they have the same bore, it's just the stroke that makes an M21 600 cc. You can only see the difference by measuring the stroke. The M20 has a stroke of 94 mm while the M21 has a stroke of 112 mm.

Henk

email (option): ahum@quicknet.nl

The inlet tract on the M21 is 1 1/6 whereas the M20 is 1 inch. Not that easy to measure accurately with the carb on.
The cylinders are interchangeable as are the cranks so the only way to be sure is to measure the stroke. After 60+ years since manufacture there could be any combination of parts.
Cheers
Pete