Okay thanks again Art! Well the one I installed's shaft with keyway on the end that the rotor sits on is smaller. But looking at the two distributors side by side the body of the smaller shafted one is a lot shorter. Smaller one 0 237 520 004 Bigger one 0 237 506 001 . Might explain why I can't get the smaller one to line up exactly as it should by 10 degrees either side. Both gears have 12 teeth but err they look a little different.....Might try repairing/ replacing hall sender in old one now. Thanks a lot will post solution.