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I need to replace bottom bracket on a 1991 Peugeot bicycle. What size should I measure when shopping for a new part? Size A is 140mm, which seems to be a rather strange size. Do I need to include the thread in the measurement?

bottom bracket

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The dimension you need to match is B in your photo. The length of a BB axle doesn't include the threaded ends.

You also need to measure the shell on your bike, though it's probably 68 mm.

See Sheldon Brown for more details. https://www.sheldonbrown.com/bbsize.html

So a fully specified cup-and-cone type BB would be something like 68x113, meaning it is intended for a bike with 68 mm shell and 113 mm long axle.

FYI: your replacement BB might have threaded holes in the ends of the axles, instead of male threads. (eg Shimano BB-UN26). You'll need bolts to go along with the new BB.

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  • Thank you. This BB seems to be BB-M500, I'm replacing cranks with FC-MT60 with bolts, and yeah I just realised it's a different mounting system. – Vladislav Ivanov Nov 14 '20 at 18:23
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    If you're replacing the crankset, the new BB should have an axle that corresponds to what the crankset requires, rather than matching the length of the existing one. It might, coincidentally, be the same, but it could be different. Have a look at Shimano's documents, especially the SI documents, there are specs in there for axle lengths. Seems like there are several variants of the FC-MT60. si.shimano.com/#/en/search/Keyword?name=FC-MT60&name_web= – Gaston Nov 14 '20 at 18:42
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    I didn't know that, thanks. I have the biopace variant, the one that seems to be described here: si.shimano.com/api/publish/storage/pdf/en/si/R-97/… I actually found a modern 122.5mm D-NL BB, so that's nice. – Vladislav Ivanov Nov 14 '20 at 18:52
  • Is it at all possible that this bike has a French threaded BB rather than a BSA, or is it too recent for that to be likely? – Weiwen Ng Nov 14 '20 at 19:01
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    Interestingly, the required D-NL is not a symmetrical bottom bracket so not just as simple as the total length of B. @Gaston. – JoeK Nov 14 '20 at 19:54

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