TRIUMPH 650 4 SPEED GEAR INTERCHANGE
Those whom have had to replace broke gears in a Triumph gear box know just how troubling it can be. First finding the gear can be a problem as only a few are still being produced new. Fortunately there are plenty of used gears still available. Though this raises a few issues with gear and shaft comparability. Like finding replacement gears were not difficult enough. In 1969 Triumph shifted all gear production from Meriden (Triumph Factory) to Birmingham (BSA Factory). Moving production also resulted in changes to how the gears where machined.
At Triumph the engineers drawings show the main and layshaft with a step or reduced diameter where the gears slide on the shaft. BSA saw this detail as time consuming to machine and revised their drawing for easier production. The time saved machining the step would make production more efficient. And our jobs today much more difficult.
Under no circumstances can small-bore gears be used on a non-stepped shaft, nor can large-bore gears be used on a stepped shaft. Care must be taken to ensure that the shafts are compatible with existing gears, or they should be replaced at the same time.
Below is a list of gears and alternate options: