Most tapped bolts are 10-32 threaded, to stay in line with Tru-Turn and the followers making machined aluminum spinners and the spinner bolts they use. For especially long spinners (distance from tip to the threaded prop bolt) Tru-Turn has long custom bolts for this purpose, but most of the time you can get lucky at your local hardware store, or custom fastener shop, such as Fastenal. Your spinner may come with a backplate that simply has an approximate 10mm hole, and some adapters to fit different crankshaft sizes. In this case there are spinner adapter kits for various crankshafts. All of this will depend on the brand, size and type of spinner you end up with, and of course, whether your engine has a threaded crankshaft, or a threaded prop bolt that attaches to a prop hub on the engine. What 26cc engine are you talking about here?