I‘m trying to replace my bottom Bracket.
I chose a Shimano UN300. Measured everything off the old one and bought the right part (i‘m pretty sure)
Now i cant get it in till the end.
The length of the thread is the same, but you can see the problem in the 4th pic.
Anyone with a idea?
by -_GOAT
6 Comments
Looks like the bottom bracket is too short.
Measure the overall length .
[deleted]
n/m I’m dumb and didn’t read it thoroughly. Are you running into the cable guide screw? Sometimes you have to back it out while installing a new BB, especially if your old one was cup-and-cone, which takes up less space in the shell.
The bad solution: throw a spacer in there so you can torque the bb properly. Your cranks will then be slightly offset from the frame centerline but it will work.
The good solution: find a bottom bracket with a shorter threaded part.
What I would do: get some new ball bearings and clean and repack the old bb
There’s one of two primary problems here (the possibility that you need a 73mm BB is a secondary problem we’ll get to). The primary issue shown is that you’re unable to thread the fixed cup of your new bottom bracket fully into your frame’s bottom bracket shell.
This is either caused by you threading in the *other* (the separate) threaded bottom bracket cup in first (it needs threaded in second and may not screw fully in), AND/OR is caused by a frame that does not have a full set of clean threads, as the old cup used less of the shell’s threads than the new cup does.
If your frame’s threads are not clean enough to allow it to go in all the way, a bike shop can “chase” them for you.
0) for brand new or recently painted/powder coated frames make sure the threads are cut all the way and are clean — a single tiny grain of grit can jam the threads.
Since you’re doing a replacement, brush/wipe the threads to double check they’re clean before greasing them. Then:
1) back the BB shell cable guide screw out
2) install the cartridge all the way, then install the opposite side/adjustable sleeve.
If the threads are chased/clean you’re either hitting the cable guide screw or the other cup.