Re: Some old bottom mount nuts have neck cracks and others don't?
The nut needs to be as tight as it needs to be to keep it from walking with trem use. If it moves so does your tuning, wildly, or just a little. UV's with 10's can be a particular pain with the slick chrome nut that doesn't want to stay put, but it's in a beefy chunk of wood, a cracked UV neck is almost unheard of. The thinner the wood, the more susceptible it is.
Wood shrinks, and continues to. Wood shrinks and all metal to wood contact loosens and needs to be maintained.
Wood also swells with atmospheric conditions. If you have a nut tight and it goes into a very humid hot day, it swells and the nut gets a lot tighter and can crack the wood where the nut wasn't too tight, when it was tightened.
Shock, case falls over, is dropped, etc. Wood with the nut tight doesn't want to be shocked.