Have been looking at the parts list for the Shock absorber.
The spare engine I obtained, which I am practicing on and rebuilding came with a two lobe gear.
The teeth are 18.
Is it right that the M20 1940 had 4 lobe gear?
This pic shows the one I have for this engine.
Seems the spring is round profile too which seems wrong. should it not be square edged?
Also it should have a pacer before it is placed onto the drive shaft I assume.
The service sheet for 1940 shows a four lobe gear.
My question if anyone can answer is can I use either, I believe it is yes. However if I wanted to keep the bike as it was, should it have a four lobe set up?
The 1940 bike would have had a 4 lobe set up..
However, this was superceded by the 2 lobe arrangement on later bikes.
So, either assembly will work OK but parts are not interchangeable between the two types (including the springs)...
The later 2 lobe assembly was introduced to give a smoother action to the operation of the shock absorber cam...
As you have deduced there should be a spacer that fits behind the shock absorber assembly...Ian
email (option): ian@wright52.plus.com
Thanks. So either will work.
The spring I have seems wrong, too big and looks like it needs a large washer to prevent it riding over the nut.
The spring is 58.25mm ouside diameter
43mm inside diameter
40 mm long
Made from 7.54mm thick steel
Maybe best to buy the full 4 lobe set, to replace it.
How many teeth are standard for M20 16 or 18.
I suppose it will effect the transmission of torque. 
A solo WD M20 has 19T on the engine and 18T on the gearbox...Ian
email (option): ian@wright52.plus.com