Thursday, December 29, 2011

Cush Drive This

The cush drive assembly on my '51 is simply spinning over under anything above moderate throttle. I think we've finally concluded it's a result of wear but I spent a fair amount of time figuring out what the correct parts should be, how it actually works and what pieces Triumph used when.

I found the following in the transmission section of the Triumph Twin Restoration book by Roy Bacon. I wish I'd found it or some other useful reference about a week earlier.

The first twins had this mounted together with a shock absorber but from 1953 this item moved to the clutch and one by one the models were with an alternator. Until this arrived those without
carried a distance piece.

All pre-unit engines had a single strand primary chain and all unit ones a duplex. After 1972 came triplex chains. Sprockets need to be checked for the fit of the crankshaft, the condition of the teeth and the smoothness of the oil seal rubbing the surface. If healthy, continue to use but if in doubt change along with the

All pre-unit models adjusted their gearing with the engines sprocket so a range running from 17 to 24 teeth was available. from 1938 to 1949 and with a 25 from 1950 on. The unit ones came in one size to suit the fixed centre application.

The 1938 sprocket was common to the range of singles and all changed in 1939. They were altered again for 1949 and to their final form in 1953 when they lost the shock absorber cam. The unit sprockets only altered with the addition or removal of dowel holes which locked the alternator rotor for ignition timing purposes. On the 650 a further change occurred for 1971 with the removal of the oil seal face and machining to allow the use of shims to set the chain alignment.

The system was only used up to 1952. With it the engine sprocket ran free on a sleeve splined internally to the crankshaft and on the outside for a slider. This and the sprocket had matching cam ramps and were held together by a spring. If the sprocket turned on the sleeve the ramp forced the slider along the splines against the spring to provide the absorbing effect.

Most of the details remained unchanged but the slide was modified for 1939 to match the sprocket change. At the same time the spring changed but the 1938 one reappeared in 1946 fitted to the new 3T. A new spring went onto the TR5 in 1949 and the rest of the range for 1950. The spring used in 1938 and for the 3T was 1 11/16 in. free length and the other 1 13/16 in. If they have shortened they should be replaced as should any other parts showing signs of wear. Minor damage may be stoned out but check that this is not finishing off the hard surface or that will make one ramp bear more load than the other.

My spring is measuring about 1 5/8. Dammit. I think I need to replace all the following:
70-1681 SLIDER, 2 LOBE,CUSH DRIVE $80.20
70-2912 CUSH SPRING $48.40

I hope I can actually find the stuff I need. I just wanna ride the damn thing...


Flathead45 said...

I didn't quite understand. What is the proper spring length for your T100?

BitMonkey said...

According to the book the 500/650's should be 1 13/16. I'm not sure if that means my spring is tired or the wrong one.