jetmech05 is correct on 4 strokes using a double nut assembly. A little blue loctite on the threads won't hurt also. Use blue loctite on 2 stroke prop nuts too. If your using a wooden prop, retorque the nuts after each days flying as the wood props crush with use and need more attention than other prop types. Prop shaft threads and nut threads must be oil free for the loctite to work, clean with alcohol and dry if necessary. Some one smarter than I here on RCU recently explained why left hand threads wouldn't work...something about the "power pulses" when the engine fires is actually in a tighting direction and if it were left hand threads it would want to lossen the prop nut in use. Makes sense as it is when a 4 stroke backfires or runs backwards that the prop will sometimes come loose. A few years ago I also had a 2 stroke shed a spinner nut and wooden prop in flight. Not tight enough, and not loctited!!