stiff bb when tightening left side

father_jack

Brand new BB, tightened right side first, but as I tighten left side, the axle starts to feel rough and can feel like the bearings are a bit shot (grinding and non smooth)

Exactly the same BB as the one I took out. Shimano UN 55 107mm 68mm English

Any ideas? Is it ok to back off the threads on the left side, until it runs smooth? I had this on another bike as well,

nicklouse

Your BB shell is not square. As you tighten the drive side against the shell end it is distorting the fang and case of the BB causing the tightening of the non drive side cup as you try and fit it.

Back the drive side off a bit. Tighten the ND cup in then back it of a smidgen tighten the drive side fully and snug the non drive side home.

Or get the BB shell faced. Only needs to be done on the side where the BB flange contacts it.

father_jack

BB are square taper so how would facing the BB shell do? Since the threads may be off also?

nicklouse

Look at the Shimano BB. see the shoulder on the drive side. (Lip) that presses on the BB. Shell when tight. And that put things of at an angle if not square.

Facing BB shells is nothing new it has been going on for years before external bearings came out.

Like I said two options. Part fit and work both sides in so the be is supported at both ends. Or get the drive side faced.

If you think the threads are in need of chasing don't worry most tools require the threads to be chased out first.

father_jack

I've backed off the left side until it feels smoother.