Hi Tim..The M33 was created simply by fitting the standard B33 engine into the M Series cycle parts...It was not an M Series bottom end and consequently all M33s/B33s have a '3 bearing' bottom end....
As the WD and later post war M Series engines are wider on the timing side due to the introduction of the air gap behind the tappet chest in the late 30s redesign of the side valves, they have the longer timing side main shaft...That and the reduction of the timing side crank cheek boss enabled the fitting of the fourth bearing into these later side valve engines...
Pre redesign M Series engines have a 3 bearing crank (1936/37 engines with the 8 bolt cylinder head) among many other differences but share virtually no parts with the M/B33...
I have never tried to fit one of the cranks from the 3 bearing M20 engines to a postwar OHV model so can't comment on whether that is possible..In fact I've never had a crank to try though I did briefly own a complete bike years back...
Maybe you used one of these earlier cranks?....
If not, then I can't see how you didn't come up against the timing side shaft issue.......Ian
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.
