Nothing much light about an M20/21. BSA were fitting the 500cc OHV engine in these frames for years. The M23 Empire Star and Silver Star and the M24 Gold Star to name a few. Ron
The M33 was a BSA model with the B33 engine fitted to the M20 cycle parts to provide more power for the sidecar men...The M20 frame is actually the heavier of the two with larger and thicker frame lugs and larger tubes in places...The M Series were the heavyweight bikes, the B Series the intermediate bikes and the C Series the lightweights...
You'll need the correct oil tank fittings and rocker feed pipe on the return side to provide a supply for the OHV rockers of the B33...The valve lifter cable will also need changing along with the exhaust system and the cylinder head steady. Engine plates are unaltered as the crankcases are dimensionally the same in terms of fitment...
Primary chaincases etc. also fit, though you will need a bigger engine sprocket..If you have the 18T WD gearbox sprocket start with a 20 or 21 tooth engine sprocket. If you have the 'civvy' 19T gearbox sprocket fitted to other models a 19T engine sprocket would be the standard one on a standard B33 engine, with a 20T/21T as an alternative..The B33 in its standard form will pull up to a 21T sprocket quite comfortably, it's really rather down to how you like your gearing.....Remember that a bigger sprocket may mean a lengthened or new primary chain as well.
If you have a 'flat bottomed' M Series petrol tank fitted you will need one from an M33...Note, not a B31 or B33, as the tank mountings are in a different position on the tanks of those models and the tank 'tunnel' is dimensionally different...
A bigger carb should ideally be fitted...1 1/8" is good for the B33 if you intend to use the separate float type..Monobloc equipped bikes had one a 1/16" smaller from memory, but do check that....Ian
