Join Date: Nov 2007
Location: santa clara, CA
Bikes: 1979 motobecane fixed, 2002 GT LTS-3 street stomper
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sounds like it's either cross threaded, or a few threads have a burr, or have stripped out: little spring shaped shavings will come out as you unscrew it if it's stripped out.
you can tighten it back up until you can remove it but be careful not to overtighten it
once removed, you can chase the threads if it's not stripped out, but once cross threaded they usually strip out when you either chase it, or re-tap it.
it is possible to bore out a BB, install a sleeve, and re-cut the threads, or bore & tap it for a custom heli-coil (STI) but this is major surgery and usually $$$$$.