What bottom bracket size ?

[andrewc]

I'm shortly going to be building up an old MTB frame. I've got a lightly used Shimano STX RC chainset in the spares box so will be fitting that. Sheldons pages tell me to use a bottom bracket with a spindle length of 110-113mm. I can get UN54 in 110mm or 113mm for £11.99 from CRC. 

Which one should I order ?  Is 2mm going to make that much difference ?

[robgul]

I asked a similar question of the forum last week .... the difference between 110 and 113 is actually only 1.5mm because the bearing case is in the centre of the axle ... and then answer was - no it didn't make any difference at all (the bike is rebuilt now)

I would go for the 113 just in case (I have one bike where a short BB caused the chainwheel to brush the rear forks - admittedly I was running a double chainset with just one ring fitted on the inside to get the chain line for a fixie)

Rob

[Dave_C]

I'm resurrecting this thread so to make it simpler to find.
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I have a Square Taper Bottom bracket 'Shimano BB-UN54 68 BC1.37 X 24' which I would like to replace.

Looking online the replacement appears to be BB-UN55? Also they are metric not imperial.

*** EDIT ***
UN26 - steel axle as opposed to CRoMo.
UN54 - 1 metal cup, 1plastic cup.
UN55 - 2 metal cups.
UN73 - 2 metal cups and a CRoMo hollow axle.
*** ***

My BB is 115mm from outside to outside.

Can anyone help my I'd a replacement please? And perhaps identify the BC, 1.37 and 24.

Many thanks.

(The BB is fine but the plastic, non drive side cup has stripped it's thread and is not serviceable.)

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[Feanor]

I have a Square Taper Bottom bracket 'Shimano BB-UN54 68 BC1.37 X 24' which I would like to replace.

...perhaps identify the BC, 1.37 and 24.

British standard thread, 1.37" dia, 24 threads per inch.
Basically the standard for most threaded road BBs.

https://www.parktool.com/blog/repair-help/bottom-bracket-standards-and-terminology

[Dave_C]

Ah, many thanks, that is the 'English thread' bit of the explanation.

I found a BB-UN55 68, 115mm on Chain Reaction Cycles which I'll order.

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