Lucas RM21 Alternator

[Martin.S]

Over the last 30 odd years I've always had a Triumph T140 and so have acquired quite a few bits and a RM21 Stator and Rotor being some of them.
The G5 I'm building doesn't have an alternator so let's see if the Lucas alternator fits - Oh it does!
Now you can read that the Lucas stator fits but you have to use the Wipac rotor - Why?
The shaft on my crank measures 3/4" as does the bore in the Lucas rotor - is something here different than everyone else's?
Cheers