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I'm trying to install a new bottom bracket (BB-UN55, BSA) into a new aluminum frame. Trying to screw them in by hand only allows me to go a few threads deep. On the non-drive side cup there is some thread locker that could explain the tightness a bit, but on the drive side, there doesn't seem anything that should block screwing it in by hand. I've checked the orientation of the threads on the shell and BB (CCW on drive side, CW on non-drive side), and the width of the bottom bracket (68mm).

Is this a clear case of needing to have the shell threads chased? Or could I proceed softly with a wrench without much risk of damaging the shell threads? What else could I check to make sure the frame and BB are compatible?

drive side non-drive side bottom bracket

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    +1 for a cautious approach and asking questions rather being 'that' Gorilla with a spanner.
    – mattnz
    May 31 '17 at 19:37
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    On an older bike I'd be quite suspicious of a mismatch in thread pitch, but I think they finally got that straightened out 20-30 years ago. May 31 '17 at 22:02
  • It's clearly not cross-threaded so proceed with caution. Make sure all threads are blown clean. Use a liberal amount of assembly grease on the BB and back it out a little bit a couple of times when installing, just enough to clear the threads if they're pushing anything that was left through. Also, make sure you've got a good tool with clean and intact interfaces and it fits well for installation. As a matter of course you should be chasing and facing the BB, headtube and brake mounts prior to rebuilding the bike. The paint job looks awesome but also thick!
    – DWGKNZ
    Sep 28 at 13:37
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If one side screws in without problems, and you've verified the thread orientation, then it's not a compatibility problem. ( http://sheldonbrown.com/bbsize.html - scroll down to "bottom bracket threading".)

Trying to gently tighten it with a wrench should be fine. If you encounter abnormal resistance, then you should start thinking about taking it to a shop for chasing.

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Maybe double-check that there is zero debris in the threads. I ran into a similar-sounding situation and it made me very nervous given the stakes. It turned out to be small bits of finely-ground rock powder in the threads. I cleaned it with degreaser, wiped it well, blew it out with compressed air, then did all that again. After, I was able to tighten it most of the way by hand as expected.

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    I tried this just now, and it was indeed a bit better after cleaning out the shell threads. I still had to screw the BB in with a wrench, and I noticed, when turning, that after it felt tight, when I kept turning, it felt loose again, up to the point where I could turn it with my hand again. My guess is that the threads of the BB and the shell are narrower at some point, and when the narrow points on both parts coincide, it gets tight... I managed to install it though, in multiple goes, checking the threads in between, and everything seemed fine. Thanks for the great anwsers!
    – bwindels
    May 31 '17 at 21:04
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I faced a similiar situation. Initial 5-6 revolutions into the shell felt easy and nice but at some point I started feeling significant resistance. I took it out, regrease the BB and the shell, and try again. This happens for a few times and eventually, little by little, I manage to thread the BB into the shell by hand. I would like to also add that at certain points when I could feel quite a lot of resistance, so I backed off a little (reverse-screw a few rounds) and continue screwing it in again. Each time it gets better until I could screw it all the way in by hand. One of the side at some point needed quite a lot of strength, but still using only hand (no wrench) gripping and turning the BB in.

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