Thanks for all the replies and especially to The Might Gryphon for pointing me to Max BMW. Excellent online fiche!
I also like TX Brick's idea of using a sealant.
I removed and inspected both the male and female threads. There is no significant damage to either. Working with the lock assembly outside of the case, I confirmed that it is extremely easy to cross-thread. Since the male threads are not full circumference, you don't sense the resistance to turning that would normally accompany a cross threaded situation. There is some but not much.
When installed, the nut will go snug against the case body and hold. Inconsistently, I will find the nut loose after several top closure and latch actions. I am beginning to think that the lid retention hook is pushing the lock body in such a way as to cause the nut to skip one thread on just the top of the lock body creating a cross thread situation after proper assembly. This would be just enough to make it loose.
This top case lid requires about the same force to latch as my 88 k75s did but that was 20+ years ago now so my memory may be failing me. Both required me to press on the lid to complete the latch. Given my theory above, I decided to adjust the latch hook to reduce the force required to latch the top in the hopes that the nut would see less force.
As lid hook appeared to be at the limit of its adjustment, I removed the hook and bent it to drop its position about 3 mm. It now takes a slight press on the lid to get the hook to latch. During this closer inspection, I also found that the hook base plate was hitting the lower case body at the seal lip so I scrapped the lip's plastic down a bit to create clearance.
Twenty open and latch cycles later, the nut is holding firm. Fingers crossed!
