Been taking apart two units to verify all up to specs and have found the pistons not meeting the part number requirements.

Would appreciate more details on the difference between the M20 & M21 sizes. As I recall the M21 is shorter. Seems the 1939 De Luxe was comparable to the M24. 66-1051

Thanks for your time as always.

email (option): unpob@yahoo.com

M20s always had the 82x94 (496cc) bore and stroke combination.(shared by the M24). Early M21s were 85x105 (596cc)and late M21s were 82x112 (591cc)... So the late M21 had the same bore as the M20 but the piston was shortened above the crown by 9mm to accomodate the longer 112mm stroke crank (conrods remained unaltered).
The early M21 had its own parts..The late M21 may well have a similar crown hieght to the OHV M24..it is also very close to the postwar OHV B33 in that regard. For that to be the case the M24(which had a shorter barrel) would have to be fitted with a different conrod to the M20/21, which I would have thought it would be....Ian

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

Glad you mention the two different storkes on the M21 as yesterday, I opened up a case to verify the flywheels as to correctness for the longer stroke and as I compared it to three of my other wheels, had to do it twice because some of the longer strokes had the crankpin located slightly shorter/longer than the other. Guessing the earlier M21 wheels, would be the closer (shorter) to the shaft.

Amazing how little I know about those bikes.,

I better go ahead and measure the stroke to verify, I may have a flywheel for the JM21.

email (option): unpob@yahoo.com

It's good to see you up at at it!

Irene must have treated you kindly.

Cheers

email (option): britool51@hotmail.com

Hey Robb, I used a bunch of my BSA motors to anchor down the house at the corners!

email (option): unpob@yahoo.com