BB size Help !


Old School Hero

I stripped down a donor bike and started rebuilding it to give a friend.
The original crankset was fitted with the BB-5500 Bottom bracket (70mm shell, ITA threading and and 109.5 spindle lenght).
I want to mount a square taper crankset, so i tried fitting a UN-55 BB (70mm shell, ITA threading, 113 length). Somehow i can't screw the cups all the way. I measured both bottom brakets from the outer edge of right cup to the outer edge of left cup and indeed UN-55 is longer by 2mm.
Both BB being designed for 70mm shells, shouldn't their cups spacing be equal ? Or does the spindle lenght make a difference ?
I'm afraid my chainline will not be right if i don't manage to screw at least the drive side all the way.

Any help will be much appreciated !



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Old School Hero
You're right it may not be that clear on the pic, but i definitely have the last thread or so still out of the shell when tightened as much as possible.
Actually when i tightened one side properly the other wouldn't go all the way (a couple threads still out after tightening), so i went for a balanced tightening so both sides are equally as close as possible to full tightening.


Senior Retro Guru
I don't understand. British cup and cone type bottom bracket bearing cups had a LH thread for the right cup which was always considered a fixed cup, that is screwed right in and very tight. The left cup is RH thread and used for adjustment, with a locking ring to hold the adjustment. This allows for some variation between the bearing surfaces on the axle. Normally used 11 1 / 4 balls each side.




Senior Retro Guru

I assume the un55 you have installed is one of those sealed unit.

I would try the following
Unscrew the left cup so that it is threaded-in only a little, but enough to act as a guide for the unit when it drifts into the bb shell. Screw the fixed cup fully home, then tighten the left cup as far as it will go.
What may have happened, is that you screwed the left cup in too far in advance of seating the right hand cup and now nothing will thread any further

If you've already considered that, try undoing the bolt that is holding the plastic guide. It doesn't have to be removed, just enough to ensure its not produding into the frame. As it is, it may be that bolt is impeding the sealed unit going in


Old School Hero

Thanks both for your replies.

@keithglos : as suggested by pigman, it's a sealed bearings BB :)
@pigman : i followed your advice and it worked. First time i have to think twice before screwing a sealed unit BB, but i'm learning everyday :) ! Still surprised at the size difference between two BB's designed for the same shell size !