My belt drive bicycle came with a Shimano internal bottom bracket. Shimano BB UN300, 127 mm length, 68 mm width, square taper. Below a picture. On the part is written

  • BB UN300
  • 68 UJ
  • BC137x24D-EL

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I bought a replacement. The body is 2mm longer. Below a picture of the replacement. The part number is EBBUN300KB27X And on the part is written

  • BB UN300 K
  • 68 VL
  • BC137x24D-EL

enter image description here

The replacement's body is about 2mm longer. All other parts seem the same size / length. The axle length is 127mm on both.

enter image description here

I installed the replacement. There is no play / seems to sit well.

  • Inside the frame: On the non drive side the bearings are 2mm further towards the outside of the frame (but still within the frame).
  • Outside the frame: the cup cannot be turned in all the way. It sticks out 2mm. Although the cup is not flush against the frame, I assume the cup is sitting against the bearing.

enter image description here enter image description here


  1. How "bad" are the 2mm?
  2. Can the frame get damaged?
  3. Will the life of the replacement be reduced?

1 Answer 1

  1. Can the frame get damaged?

Yes. However, the frame can get damaged even with a shorter bottom bracket that would allow the cup to be fully threaded.

The bottom bracket threads are a marginal retention mechanism. A usual engineering rule is that if you see a left-hand thread somewhere, it's a faulty design. This means bicycle crank arm pedal threads and bottom bracket threads are faulty.

The crank arm pedal threads should have a countersink, with a conical surface in the pedals. This would allow eliminating the left-hand thread. It would also prevent crank arm failures near the pedal eye.

The bottom bracket shell should be smooth without threads, and it should be split in the same way threadless stems are split. The bottom bracket would be pushed inside the shell, and the shell would have several bolts that would be tightened to secure the mechanism.

So, any bottom bracket shell will fail, its threads will become destroyed. How this happens is a question of two factors:

  1. Is your bottom bracket shell steel or aluminum (even steel will fail, but aluminum will fail faster)
  2. How much thread retention you have (less thread retention would destroy it faster)
  1. Will the life of the replacement be reduced?

I would be more worried about your frame. Bottom brackets are cheap and disposable.

However, I think most forces would go through the bottom bracket cartridge side, not through the lockring cup side. So maybe it will work just fine, as fine as the poor thread retention mechanism can be.

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