The best solution is to have your LBS chase the bottom bracket shell's threads. Remember, on a decent frame, the bottom bracket should thread into the bottom bracket shell by hand. Having said that I can say that over the years I've had an occasional bit of trouble with Shimano's sealed bottom brackets. And if the non-drive cup is plastic, don't use it...get a metal one, and, as has been said, use lots of lube. Good luck!