I wanted to know if the two parts of the cush drive should be able to rotate past each other? On mine the spring coils bind solid just at the tops of the cam ramps.I did fit a 3mm spacer behind the inner sleeve to align the primary chain and wondered if this affects the way the cush drive operates?

Many Thanks Pat

Hello Pat,

If I remember correctly, there are 2 types of sprockets:
One with a casted spacer on it, and another which requires a spacer,
I guess kind of like the one you put.

May be the spacer is a little too narrow (I think it should be some 4-5mm)
So it lets the cush drive pass the peaks.

I also had it happening under engine strain and acceleration In one point
Though all was there, so I put in a different spring, I think It was a clutch spring, which
Did not squeeze as much as the other one, so it did not let the cush peaks to pass each other.

Noam.

email (option): noam10@gmail.com

I don’t think you should deliberately set up your cush-drive so that it locks up before the cams override each other. This and similar mechanisms were designed (I believe) so that the cams are able to override each other in extremis. The device has a dual purpose - mostly it’s there to even out torque fluctuations and provide snatch-free transmission, but it also prevents chains or the crankshaft itself breaking when subjected to violent loads, such as the rear wheel suddenly locking up or dropping the clutch with the engine screaming. I’d rather have the cams clattering over each other than risk a broken crankshaft!

ChrisJ

Hi Chris,

I do understand the logic behind what you are saying,
But when it slips, it would slip also when you gear up / down, accelerating or going uphill to some extent, and wear itself
And the crankshaft if too “soft”.

The system is there in order to protect the drivetrain from sudden violent torque blows.
And (as far as I know) did not mean to slip over.

More modern bikes have a rubber flower petals shaped system to take the blow inside the rear wheel hub,
And Triumphs has a setup in the clutch system, none made up to slip.

Noam.

email (option): noam10@gmail.com

The assembly is a 'cush drive' and is only there to smooth out power take up. When correctly set up one cam should rise approx. a third of the way up the other under load....not more. The spring should not be 'coil bound' at any point during normal operation but should have sufficient pressure to restrict the cams movement to the limits described and thus prevent the cams 'ratcheting' over each other...If the spring is weak and the cams actually reach a situation where they are 'point to point' they will frequently jam in that position.
Later 2 lobe versions have an 'easier' action due to the increased cam radii but are fitted with a spring with greater pressure to compensate. Flat wire (4 lobe) and round wire (2 lobe) springs should not therefore be interchanged..Like girder fork springs, valve springs and anti drain valve springs, the cush drive spring won't last for ever and if it is not functioning correctly it should, ideally, be renewed. Increasing the preload with a collar fitted under the cush drive spring nut is a 'dodge' to get out of trouble, but is not a method that should be used if the result is a coil bound spring......Ian

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

The spring in mine was NOS square wire from a reputable source 5000 miles ago. It should still be OK ? As Ian said on investigating the cams were locked on the tops and one had chipped 2mm off the end. Standing on the kick start against compression takes the cam 1/3rd of the way up the ramps.

Cheers Pat

Hi Pat..The spring should be 1 5/8" long when new and to compress it to a length of 1" should require a force of 170-190lbs...It's a 'fail' if that test requires 160lbs or less..
I have come across various different length springs that conform to the other relevant dimensions..I suspect these were used on other models with different requirements...However, BSA had such a multitude of models through the 30's/40's/50's I have no idea which is which . I would expect 50,000 miles as an absolute minimum on a new spring, before even thinking about checking it..I have done considerably more than that without problems. Going back to your original posting if you put an additional spacer behind the crank sleeve this shouldn't affect anything..The relative positions of the sprocket, cams, the outer end of the sleeve and the spring location shoulder on the nut remain unaltered, you have just moved the entire assembly further along the shaft. So spring pressure would not be altered..To pre load the spring you need a spacer fitted over the shoulder on the nut where the outer end of the spring locates..Ian

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