I recently built up an old Trek 520, and swapped in a cartridge BB. It's a Shimano unit, I believe a UN-72. At any rate, the drive side threads are fixed to the cartridge. The crankset is a Shimano B-124 triple.

When I built it up, I had a couple mm of clearance between the small ring (which I never use) and the chainstay. The driveside thread flange was torqued down tight against the BB shell.

I am guessing I've ridden the bike a couple hundred miles since build-up. Everything was great until yesterday, when the small ring started rubbing on the stay.

There is no play detectable in the BB/crankarm assembly. The bike hasn't been crashed or abused in any way. I am at a loss to think of any explanation other than the spindle shifted a bit toward the NDS. Is that even possible? I tried a google search but couldn't find a reference. Any other ideas?

As for fixing the problem, I will either go back to the OEM cup/cone unit, or add a spacer to the cartridge unit. For convenience I would prefer the latter, but if there is reason to believe I have a faulty cartridge, obviously I don't want to re-use it.

Thanks.