Hi All, hoping someone can enlighten me on this subject.

My question is: Should the cush drive spring fully close (locked coils) before the cush drives lobes override, thus preventing the lobes overriding or not?

When I tighten the cush drive nut fully to the end of the thread on the drive shaft, the lobes overide. I made up a 1/8" spacer (washer). If I fit the spacer under the spring, I prevent the lobes from overriding.

So putting my question another way: Do I fit the 1/8" spacer or not.

email (option): pvlietstra@gmail.com

The cams (lobes) should not over ride each other during normal operation...

If you press slowly down on the kick start the cams should move about a third of the way up each other before the increasing spring pressure overcomes compression pressure and the engine turns over through compression...

If you have the wrong spring or a weak spring, fitting a spacer is OK to solve the problem as long as it restores correct function...

Worth noting is that 2 lobe shock absorbers use a stronger spring made from round wire, 4 lobe assemblies use a weaker spring made from flat section wire..

The 2 lobe cams have much larger radii at the base and peak than the 4 lobe cams to smooth out the 'action' of the shock absorber...These 2 lobe cams move more easily against each other, thus the stronger spring to prevent movement until the correct load is applied...Ian

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

Thanks Ian for your comments. I have a 5 lobe cush drive and a spring with a square wire section. The lobes do not override when I kick the engine over and only move about 1/4 of the full travel. They do however overide when I tighten the drive shaft nut, unless I fit the 1/8" spacer, which then locks the coil before the override position is reached. Do you think I need to fit this spacer?

email (option): pvlietstra@gmail.com

['I have a 5 lobe cush drive']...

Sorry, I assumed we were talking about an M20...These have 2 or 4 lobe cams..

If you do have an M20 with 5 lobe cams I have no idea what machine they came from...

If that is the case or if it's not an M20, then the shock absorber set up will be dimensionally different and my comments wouldn't necessarily apply....Ian

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

Ian, I appreciate your comments and am sure you are correct. However, I bought the cush drive through this forum in the "Parts for sale/wanted" section. It was sold to me as a M20 cush drive. The splines fit, the sprocket lines up with the clutch srocket and the nut fits the drive shaft, so I will be using it. I am just not sure whether the lobes should override when I tighten the drive shaft nut or not.

email (option): pvlietstra@gmail.com

It's quite possible that the shock absorber came off another BSA but will fit the M20...

BSA produced multiple models with similar shock absorber arrangements....

The only certain thing with a 5 lobe set up is it isn't an M20 one from 1938 onwards..Perhaps, it is from the very early 36/37 version..I'm not sure whether that is the same as later M20s...

Anyway...The lobes should not ride over each other with the nut fully tightened...Ian

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

I must be loosing something. Old age is creeping up. I counted the lobes by rotating the shaft and must have counted the first one twice. I have now taken the cush drive off again and counted the lobes properly. It has 4!

When I bought the cush drive, the nut was ropey as nuts tend to get when fastened without proper tools. I found another nut at a swop meet. I have discovered that it is 1/8" thicker than the original, hence my need for a 1/8" spacer.

Thanks for your help and also your prompting to go a check the number of lobes!

email (option): pvlietstra@gmail.com

Mystery solved!... ....Ian

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

If the replacement nut is 1/8" thicker than the original then surely an extra 1/8" spacer is the last thing you need. I would machine 1/8" off the nut.

Thanks Ian for the clarification on the round/flat springs. Had them for years and did not know why

email (option): unpob@yahoo.com