Just bought a 56 ZA which came without an advance & retard pipe fitted. the vendor sent me a second-hand item which I spent a couple of hours de rusting and painting before attempting to fit it. Distributor end fitted fine but having spent 2 hours struggling to fit it to the tapped hole in the rear carb body came to the conclusion that the male threaded nut seems to be slightly larger than the tapped hole. Is there a difference between a ZA and a ZB and perhaps they sent me the wrong one? Any advice as to how to resolve the issue appreciated.