I don't think I have done anything out of the ordinary, just a 3/8 UNF insert melted into the wheel centre which screws up to the shoulder on the gearbox output shaft. This is then retained by the left hand threaded screw and a flat washer. I'll get a few pics up when I have chance. Hopefully I was just unlucky but the break in the screw was almost level with the end of the gearbox shaft hole, so could be tricky to remove.