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My bike has recently developed creaking noises from the bottom bracket area so I decided to take everything apart, give it a good clean, apply some lube and reassemble.

What turned out to live inside the bottom bracket shell was BB-UN100 with plastic cups which I promptly ruined on reassembly. So I am looking for something more sturdy and preferably not made of plasticine.

The spec for my bike shows that it normally comes with BB-UN26, mine for whatever reason did not, but from the picture it seems that at least one cup it comes with is made of plastic.

My BB-UN100 has the following etched on it:

  • BC 1.37x24
  • LL123 BC 68

I gather that 68 refers to the shell width, which is 68mm indeed, 123 to the axle length which a quick glance at a vernier caliper confirmed to be 123mm, and the thread type is English. No idea what LL signifies. I came across some manual that said it is a code for 113mm spindle length, so I'm a bit confused.

BB-UN55 looks like what I want and is available in 68 122.5mm size. Will it fit? The fact that they bother to specify the axle length to a tenth of a millimetre (the only length like that) makes me nervous.

Otherwise what other options do I have?

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Any square taper bottom bracket with BSA threads (1.37inch by 24tpi), 68mm shell width, JIS taper and axle length roughly 123mm will work.

Changing axle length from 122.5mm to 123mm makes 0.25mm difference to chainline, which is unnoticeable. Many stores list BB-UN100 bottom bracket as 122.5mm, so it is possible that there is actually no change at all. I could not find any Shimano documentation for the letter codes, seems that Bartnett's explanation does not match reality.

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  • I don't understand how going for an axle that is 0.5mm shorter from what was factory fitted is "wrong" while getting an axle that is 10mm shorter is the right thing to do?
    – Johnny Baloney
    Sep 14, 2019 at 20:05
  • Uh... I was careless when reading the question. 122.5 is indeed the right choice, and the 0.25mm chainline difference will be unnoticeable. I'll edit the answer.
    – ojs
    Sep 14, 2019 at 21:02

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