5/8-3/8 Chain Conversion by Rickard Nebrer
Front
Use a sprocket from a BSA A65 and grind of some part of the inside. (look at your old sprocket)
Rear
Use a ordinary rear sprocket from a BSA A65 or a Triumph from 1971-1973.NOTE
It should be remembered that the heavier the chain the greater the mass of and the greater the power losses in it at high chain speed. If I can remember correctly if you double the weight you quadruple the 'flying' mass. Wnen many years ago I was deciding as to which chain to use on the rear of a so called 'classic racing' Norton Atlas I gave our local Renold office a call (they had them in those days) and was told.'You can use the 5/8 x 3/8 if you want to waste a couple of extra horse power in the chain but I would suggest you stick with the 5/8 x 1/4 as Nortons did for years and swap it for a cleaned and regreased one every meeting.' The gent also pointed out that the 5/8 x 1/4 chain size was originally made for motorcycle applications in the distant past as it was a compromise of strength and weight. DO NOT go looking for motorcycle chain power losses test results, you will be wasting your time and I have picked the brains of everyone I can find including retired ex Renold gents and none can recall any such testing ever being done. If you know different please let me know. Some brain deads even use heavier 3/4 pitch chain on the rear of thier British bikes.....its stronger......wow......the power losses will be a lot greater....... and of course there will be less teeth in mesh at any time on the gearbox sprocket and being a larger pitch the greater the chordial vibrations introduced which also sap power.....J M Leadbeater