S/A Cush spring

[Gino]

So the good news is that is that after putting in an electronic ignition and “butchering “ the Cush drive I have a bike that goes very nicely, so happy when it fired up .Happy BSA owner now. I will post videos soon

[Petergj]

Hopefully folks will not mind me re-opening this topic but my 1958 A10 Golden Flash restoration continues. After my euphoria of installing the complete engine in the frame, I came to fit the primary cush drive assembly. Bearing in mind the bike was purchased as a basket case, the drive components were loose and I was surprised to find the cush drive spring was virtually coil bound when the nut was fastened. When the spring was removed and the nut tightened on the crankshaft, the Crush Drive Cam cannot travel over the top of the lobes (maximum movement is 3.4mm). On checking the distance between the nut and Cush Drive Cam for the spring to fit under static conditions, the gap was approx 30mm (1.18") with the old sprocket and 27.5 mm (1.08") with a new sprocket (which has a 0.100" shoulder). So two questions :-

1. I assume the spring I have (length - 1.674"/42.52mm, diameter - 2.349"/59.69mm, coil diameter - 0.300"/7.64mm, 4 coils) is totally incorrect and I need 67-1136?

2. The smaller gap for the spring to fit using the new sprocket with its shoulder will increase the spring preload - how critical is this? I am not considering machining the shoulder off the sprocket as the chain alignment is good.

Any comments appreciated and thanks in advance.

Regards,
Peter

[chaterlea25]

Hi Peter,
You have the wrong spring
It should have 3 and 1/4 turns

As far as I know the M20's had the 4 turn spring.
A read of my posts on primary setting up is linked further up this topic.
Various engine sprockets have different or no rear protrusion.
Make sure the engine and clutch sprockets are properly in line as a preliminary to building the primary side.

John