A Lucas RM20 replacement rotor. This rotor has an outside diameter of 74mm and is suitable for use with the RM21 stator also sold on this site. The 70mm rotors are no longer available.
This has the standard 3/4 inch mounting size. The earlier (pre 1962) Tiger Cubs with the RM13 rotor had a smaller mounting size, and this was overcome by Triumph using a spacer (E4458) and an adapter sleeve (E4454) on the end of the crankshaft. Unfortunately I do not have these additional parts available at the moment.
The Lucas part number is LU54202299