Originally Posted by
verktyg
...Fixed cups loosening while riding was more common during the 1970's when components and frame quality was lower. Undersized threads on the cups and oversized BB threads, especially when they'd been chased by someone who didn't have good tools or didn't know how to use them resulted in loose fits.
I have had and have seen fixed cups on French bikes come loose far more often than on Italian bikes....
In my experience, equally important is the alignment of the thread cut with the axis of the shell and the squareness of the shell face. The greater the deviation of the thread cut and/or shell face relative to the axis, the smaller the contact area between the cup and shell face.This decreases the level of static friction necessary to resist the hypocyclic fretting precession. Of course the the thread profile and and tightness also factors, but the quality of the interaction with the shell face sis often overlooked.