For the past few years we had had issues with finding BSA crankshaft rotor nuts that actually fit. We have encountered rotor nuts that were too small and too large with the incorrect thread specifications 

Not only are some rotor nuts made incorrectly, but BSA changed the nut “specification” without changing the part number! From what we know, there was a spec change on the rotor that happened around 1968ish

Early types where 5/8″ X 20 TPI (British) and the later types where 5/8″ X 20 (American) – although on paper they look the same, in fact they are not. The pitch diameter is different from British threads and American 

 Each nut is machined here in the USA – Measures 5/8″ X 20 (British) designed to work with A50 / A65 models up to approx 1968 and also fits some early BSA C15 / B40 models that use a rotor nut without a flange…. Check your parts book and measure your cranks