Both great answers! It is a Dave Brown Spinner. I also thought the spinner itself may be sliding because I use a dynatron starter. Perhaps there is a large spinner that won't require the bolt in the center. If so, I can just use the bolt that came with the prop hub. I also should have thought about making a reference line to see what is moving. I'll make a few marks, run the motor and see if I can figure out what is going on.