This is the second time I've stripped threads off of a 170 crankshaft. What happened in both cases is the inner nut gets bad threads and starts going round, taking the crankshaft threads with it. There follows great havoc in its wake - getting a busted nut off means holding the engine with big pliers while forcing the nut off. If you're lucky, only the crankshaft thread damage is left, if you're unlucky the spinner/backplate/prop washer and related parts are busted too.

What to do? I'm thinking the nuts are the problem, they are just plain lousy. I'm not sure of the brands but I've busted both tru-turn and original YS nuts. It's possible to get the crankshaft threads back into shape or avoid the problem area by using a thicker prop washer but which nut/lock nut combo do people use?

I'm thinking the best bet is the Tru-turn double jam nut - the one that extends into the prop. Then you have a much wider set of threads meaning it's less likely to come loose and less likely to bust threads. Problem is, this ##¤%&Q! nut extends about 5mm into the prop hub itself and is a whopping 13mm wide, meaning you have to ream away much of the prop hub. Is this safe? Which tool is best for such a job? Are there other _good_ nut/jam nut combos I don't know about?