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Re: Visual differences between an M20 and M21 crankshaft

Both engines use the same conrod and both use the same flywheel set but machined differently to give the different strokes...If you measure from the centre of the big end pin to the centre of the mainshaft it will measure at 56mm for the M21 and 47mm for the M20 (both figures are half the stroke of the respective engines with 112mm for the M21 and 94mm for the M20)
One other possible difference is the material the flywheels are made from...In different years both models used either steel or cast iron flywheels...All military M20s are the steel type...If the flywheel numbers commence with a '6', e.g. 66-635 the flywheels are steel ...If the number commences with a '7' or a completely different number then they are cast iron...Steel would be the preferred choice but if you are building a standard engine cast iron will serve OK....Ian

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

Re: Visual differences between an M20 and M21 crankshaft

Many thanks Ian, you have confirmed my suspicions i.e. I have an M20 crank not an M21

email (option): steve@bsa-inc.com

Re: Visual differences between an M20 and M21 crankshaft

This is what my M21 crank looks like. (Ian prepared it many years ago) Ron

M20-028

email (option): ronpier@talk21.com

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