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I think my bottom bracket has worn and I would like to replace it. I was looking at the Shimano 105 BB. BikePedia lists the size as 68mm, which is fine, but there seems to be another dimension. I can get 68 x 109 mm or 68 x 118 mm. Is it possible to work out which size I need without removing the current BB?

update I'm not sure which of the below answers is correct. In the end I bought a SM-BB6700 Bottom Bracket BC1.37 which worked fine (techdocs here).

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This is a good question - but too specialised (sic). How about generalising the question - "How to calculate the bottom bracket size?" - then it would be more use for more people. – Unsliced Apr 26 '12 at 8:24
@Unsliced: I think this question is acceptable as is, but your question could be valuable on its own. Why don't you ask it? – zenbike♦ Apr 26 '12 at 10:11

2 Answers

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It's nominally the distance from one end of the shaft to the other, with the cranks removed (though there can be a 1-2mm variation). You can rig a caliper to measure it with the cartridge in place, or you can "eyeball" it with a ruler.

But once you have the cranks removed it's not that big a deal to remove the cartridge. (Still a little tricky to measure with the cartridge removed, though.)

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It is best to remove your current bottom bracket to check, but as a rule of thumb, a triple (3 gear) front crank uses a 118mm. A double (2 gear) front crank uses a 109mm.

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