Clean the threads.

Lockite the fan after tightening it with a crank lock tool. The make sure the prop nut is tight, use the aforementioned crank lock and a socket wrench.

Here's the important part, let the locktite cure 24 hours before you try to start it.

Also clamp your fuel line till you are ready to start the engine to keep from hydro locking which will make it spit the fan.