I have heard of a project bike for sale. It is a WM20 with a motor in very bad condition (water got in to motor). The rest of the bike is supposed to be OK. A 1949 ZB34 motor was bought to replace the WM20 motor many years ago and the owner died before anything was done.
So will the ZB34 motor drop in to the frame without issue or will more work need to be done? The early post war M series and B series used different frames but they still had the same gearbox and primary................I think?
You'll need an M33 head steady, the OHV valve lifter cable, a rigid M33 exhaust system, a larger carb with appropriate jetting,the parts to modify the oil return pipe to include a rocker feed oil supply, including the rocker feed pipe (as M33/Rigid B33) and an M33 or later M20 (1947-53) petrol tank (many of which have the 'indent' for the OHV engine)...However, not the rigid B31/33 tank as this has a different tunnel angle and front mounting brackets....Ian
The B33 engine was fitted by the factory, the result was called an M33. An alloy engine may need a different head steady, for starters. I had thought all the 34s were alloy
The iron engined B34 (competition) commenced at XB34 101 in 1948 and continued in 1949 at engine number ZB34 101. From engine number ZB34 2001 in 1950, the alloy engine option was introduced with the ZB34 'A' suffix denoting aluminium top half and starting at engine number 2001...However, the 'A' engine was the option, so the iron unit continued for a few more years...These are competition B Series engines, not Gold Stars...Ian
Take a close look at the barrel at the front and rear between the head bolts..It's not unknown for the alloy 500 barrels to crack there...They can be repaired but the liner has to be removed, the barrel welded inside and out and then remachined before refitting a liner...Not the most straghtforward job!!...If welded on the outside only they will usually crack again...Ian
