I am starting the process of rebuilding a prewar 1938 handchange M20 and wondered if there are any design differences between the 1938 M20 engine/gearbox and the later M20 engine/gearbox used through WW2. The supply of parts for the 40's engines and gearbox looks good and I was hoping most parts are the same.

Any comments on the above appreciated..

dave

I expect Ian will give you chapter and verse. But is that the narrower engine without the air gap behind the tappet chest and 8 stud head? Or is that an earlier feature? Ron

email (option): ronpier@talk21.com

Did they also run a single bearing on the timing side?

email (option): davmax@ntlworld.com

The early type does have the narrower crankcase..also the cylinder with no airgap behind the tappet chest. One bearing only on the timing side with a shorter mainshaft to match.
There is no 'outrigger plate' supporting the cam spindles..these fit directly into the back of the (different) timing cover. The tappet cover is also different. Cylinder heads have the straight 'fore and aft' fins with only 8 cylinder head bolts. Most gaskets are different. Instead of the one piece valve guides of the later engines this one had two piece guides.
In fact few parts are directly interchangeable and the later engine will not fit into the early frame either as this is narrower to suit the crankcases..Gearboxes also have some dimensional differences in the shafts and gears.Ian

email (option): ian@wright52.plus.com

Hi Ron, I am afraid all I know at the moment is that its certainly a 1938 engine, I am away from home at the moment, so cant take a good look at it.. I just wondered if there were any design changes, so from your answer I guess there are. So I had better get the parts books out and start looking at numbers..
Thanks for coming back on this... I have quite a rebuild ahead of me, so I guess I will have a few more questions in the future.

Rgds dave

Ian, thanks for that info. Considerable engine design differences then, good job that the engine is in very good condition and I have the parts book coming. Guess its going to be a case of comparing part numbers to see what is the same. Luckily I have 99% of the bike, I am missing all of the rear chain guard and also the rear numberplate, plus the footbrake pedal ( which I know is different in design ) but thats mainly it..

Rgds Dave

Hi. I use tappet cover from '38 engine on my '40 engine. It is exactly same. With piled arms logo. '37 was without logo but probably same. Is oilpump same too?

Kalle

Kalle same pump as later engines,I had to replace the bottom plate on the pump on my HM21 as someone had used a piece of copper pipe as a pickup with out the ball valve fiited Dave