Nut - Propshaft Coupling Nylok Nut
Many ski boat have the SEF Brass shaft coupling, this coupling has a castle nut with a cotter-pin "curled" into place to keep it from loosening. This system works but is difficult to assemble when on the bench, and still more difficult to remove when on the bench. To remove when the shaft is in the boat is VERY difficult but can be done. In our shop I never re-use the castle nut/cotter-pin. I replace them with a single Stainless Steel Nylok nut, however this is a special "short" nut. The short or shallow nut does not need a cotter-pin. These are not easy to find and worth the price!
Thread is 3/4-10 (same as prop end only )