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I have this bottom bracket on my belt drive bicycle.

Shimano UN300 68 UJ BC137x24 D-EL

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The person who sold me the bike said the BB's length is 127mm, width 68mm, and square type. (I cannot get hold of the seller because he is in a far away country.)

When I went to a bicycle shop, they showed me a replacement. It was non-Shimano and printed on the BB was 127.5mm. (I forgot to take a picture :( )

I looked on Shimanos website and 2 random online shops.

Shimano and one of the shops have 127mm. The other site has 127.5mm

Random shop 1 https://www.rosebikes.de/shimano-bb-un300-bsa-68-vierkant-innenlager-2688343

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Shimano https://bike.shimano.com/en-US/product/component/tourney-tz500/BB-UN300.html

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Random shop 2 -> 127.5mm (and it has the D-EL code) https://www.bike24.com/p2372410.html

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Question

  1. Is 127mm = 127.5mm, i.e. are these one and the same part and some people are just lazy and leaving out the .5mm?
  2. OR Are there 2 BBs, the one is 127mm and the other is 127.5mm? And only 127mm will be the correct match for my belt drive?
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  • Is this for a Gates belt drive? If so either spec will keep you in range for the belt line. Have a read of this answer bicycles.stackexchange.com/questions/67240/…
    – Warren Burton
    Commented Jun 7 at 19:31
  • When you have a belt drive, the alignment of the belt is very important. Replace the bottom bracket with the exact same unit as was in the bike previously. I will provide a better answer later unless someone chooses to chime in with a good one.
    – Noise
    Commented Jun 7 at 19:34
  • Also see bicycles.stackexchange.com/questions/84511/…
    – Noise
    Commented Jun 7 at 19:39

1 Answer 1

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I have a new Shimano BBUN-300 68x122 D-NL. Measured with a caliper the axle width it is in fact 123.3mm. I also have an older used BBUN300 which measures excactly 123mm.

Both are supposedly 122mm, so it might be that the manufacturer starts with the same precursor part for both D-NL (122mm) and D-LL(123mm) and machine the ends differently to get the final intended chainlines.

The origin of the "127.5" might be an actual measurement done on the axle since this value is larger than the widest (127mm) axle Shimano lists, and it fits with my axles being wider than the stated length according to Shimano's website and the packaging.

So my guess is that there is only one Shimano 68x127 D-EL which is listed as both 127mm and 127.5mm and that it is in fact slightly wider than 127mm if you do a measurement.

Replacing the original part with the same part would be the safest option.

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