There is no adapter nut used on gas engines. You will have to drill the spinner back-plate the same as the prop. The front of the spinner is held on with a 5mm Socket Head Cap Screw (SHCS) or a 10-32 SHCS that goes thru the front of the spinner into a threaded hole in the center of the prop shaft. Do not over-tighten this SHCS or you will warp the back-plate. BTW For the best deal I've found on really nice aluminum spinners (see photo) check out Cermark. The 3 1/2" is like $20.00. Be sure and tell them it's for a 10mm shaft so they send you the right bushing.

http://www.cermark.net/List.php?TID=50