I have posted several questions, being somewhat ignorant.
I have done some reading and I believe post 1949? they all used the same crank, con rod and bore. The crank pin location in the flywheel and gudgeon pin to piston top determining the stroke and the capacity?
Is this right, does this mean to convert from M21 to M20 only the appropriate crank and piston is needed. Gearbox will be wrong of course.
Thanks
Matthew
I think that the only difference between a M20 and M21 is the crank. The piston barrel and head are all the same.
Quote from: watts on 19 Jul, 2025, 16:35 I have posted several questions, being somewhat ignorant.
I have done some reading and I believe post 1949? they all used the same crank, con rod and bore. The crank pin location in the flywheel and gudgeon pin to piston top determining the stroke and the capacity?
I have a B33 and it has a M20 crank , that apparently has had the timing side shaft changed and re riveted to the b33 one . Cheers
Is this right, does this mean to convert from M21 to M20 only the appropriate crank and piston is needed. Gearbox will be wrong of course.
Thanks
Matthew
The M20 and M21 barrel are same. The cranks are different. idie reckons they have the same pistons but the M20 piston is part no. 66-1193 whereas the M21 is 66-1215. I've no idea what the difference is, they use the same piston rings.
The piston difference is the position of the gudgeon pin which is higher up than the M20 item, were this not the case it would stick out the top of the cylinder due to the longer stroke of the M21.
Quote from: JulianS on 20 Jul, 2025, 12:33 The piston difference is the position of the gudgeon pin which is higher up than the M20 item, were this not the case it would stick out the top of the cylinder due to the longer stroke of the M21.
Makes sense.
Doh! Of course the pistons are different.
I think that all makes sense.
Thank you
Matthew