Originally Posted by Andrew R Stewart

Loose ball cups can crack around the ball track after many miles of abuse, and an incorrectly adjusted Bb is abuse. The ones I usually see can be unthreaded from the shell with standard cup tools. Rarely the cup's end will completely separate from the threaded portion. The shell's threads are usually not badly harmed.

Thanks for this info, Andy, and I hope you're right - visually the threads look bit shallower to me, but they might still have enough bite for a new BB. I can only guess that I cross-threaded the cup the last time I o/h the BB (about three months ago). Idk how that happened, I've done this a zillion times on a variety of bikes with different cups and tools and never had an issue before. Maybe I was rushing or distracted that day, idk.


The cup will become distorted after cracking, the end will look cocked WRT the threaded portion. Trying to reuse or even reinstall a cracked cup is bad decision making.

I realize what I saw as cockeyed screwing the old broken cup back in was at least partially due to the flange of the cup being off-kilter. But I think the resistance I felt and some wonky movement tells me there's some BB shell damage also. I only screwed the old cup in until it felt rough, then backed out and removed it. I sprayed the shell with WD40 first and wiped it down as best I could.


When chasing a BB shell threads it is very important to maintain the shell's sides' coaxialness. So only chasing one side without the other side's tap in place, and support the pilot shaft, is another example of poor decision making. If a shop or builder is willing to only run one side's tap through the shell then question their experience. It is very easy with such a large diameter but fine pitch tap to miss align the threads and cut across the OEM threads. Andy.