Sorry, guys. Dont know the specific cause, but I just incase…I have always tightened my prop nut down to firm by holding the blade by hand, then placed a block of wood between the prop and the lower unit and tighten it down to the next even tab or flat on the lock washer. It is never too loose that way, and as long as I keep a good breaker bar in the boat (I do) I can still use that adrenalin surge to break it loose to change a prop on the river while standing on a sandbar if I need to. –Bob La Londe Forums, Free Lures, Product Reviewswww.YumaBassMan.com CNC Molds N Stuffwww.CNCMOLDS.com