The best way to secure the prop nut is to use a lock nut ahead of it..... Let me see if I can explain that a better way.

After you put the prop on you add a thin standard nut like the one in this link...

http://www2.towerhobbies.com/cgi-bin...?&I=LXL742&P=0

Then you put you prop nut on over that and the two work against each other to stay tight. Four stroke motors usually are more likely to throw props than two strokes. I think it has to do with backfiring in the cylinder. Hope this helps some.[8D]