3/4 inch seems to be all that will fit in the hole. Mine is 3/4" and barely squeezes in there. I used a drawing that someone sent me to cut it out of plate steel, but if I did it over again, I would move the lifting block aft a little for better purchase. The pivot point (possibly the angle below it) seems a little off as well as the keel does not swing all the way forward and dangles from the bolt. But what do you want in a toy boat? It works well enough.