Just looked through my notes of the build of the M33 motor that I was to fit into my XM20.
The original flywheels were poor as i could not align them well enough due to damaged crank pin tapers, these flywheels has a casting number of 66-635. A set came up on eBay with the same casting number and they were advertised as M33, they were around 40 miles away and i picked them up. They looked good and visually the same as my damaged ones. I fitted my rod and pin and they trued up nice but never measured the pin to the mainshaft measurements. Only when i fitted the piston and cylinder did i find out they were M20 stroke being 6mm longer than the M33. Easy to sort out by turning 3mm off the bottom of the piston and a 3mm packing under the barrel.
So i was lucky for once, and as you say they must be early pre war M20 with the short timing shaft, maybe that is why they were advertised as M33/B33. Flywheels with casting number 66-635 could have been bored to any half stroke centres, the only other markings on the flywheels were a single letter stamping being a letter K on both flywheels. When i built the motor i had no idea about the different lengths of timing side shafts, so as i said a bit of luck and a nice powerful motor for a girder fork bike.
Thanks for explaining the early engines.
