OK to move from 120mm to 122.5?

[Cheule]

Hi folks, I have an old cup and cone style BB and want to replace it with a sealed unit, maybe a Shimano UN54 or something like that. I shouldn't have any problems moving to 122.5mm should I? Can't seem to find 120mm modern BB's round my way.

[RobAR]

Cheule, you want to first look at the clearance between your chainrings and the chainstays. If you have more than 7 or 8 mm clearance, I would recommend going with a 118 which is more readily available than a 122 and may offer better chainline. If you must go 122, ensure that your front drlr will move outboard far enough by loosening the HI limit and pushing it out. With the small amount that you're talking about, I don't foresee any problems but... you know the drill.

[trevgbb]

First you need to know your BB width, commonest is 68mm, then 70mm and 73mm. you need to buy to suit this.
The overall length is from face to face and does not include any thread if present, not seen a 120 before, 118 and 122,5 yes. Suggest you re-measure to be absolutely sure of your size.

The other important size is from the top of the bearing collar on the right side, this should be the same as the old one, any shorter and the crank arms may foul the frame, any longer and you will lose your chain line and your FD may not cope.

The left side is not so important as long as it is not shorter.

Note that axles usually have a size stamped on, this one is 3ss and is 116mm. long and is for a 68mm. bb. you can clearly see the rh is longer than the lh, although they can be the same on other models, if stamped 5x it is for a 70mm. bb and 7x is for a 73mm bb.

I would be very wary about changing unless you are sure you will not have the problems mentioned.

I personally wouldn't bother with a sealed unit, but if you do, do not use one with delrin threaded sleeves as these have a nasty habit of breaking up when you try to remove them.