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I need to replace my bottom bracket cartridge because it is knackered. After a bit of reading it seems that there may be many types of cartridge variations (axle lengths, threads, size of square taper, etc etc). So I'm not sure how to fully specify the required replacement so I can be certain it will fit (other than buying the exact same part).

The old part is marked as a Stronglight JP400 122mm. Chainset is shimano. Bottom bracket shell width is about 69mm (as near as I can measure it). Frame is aluminium.

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From what you say, you need a bottom bracket with:

  • length = 122mm (as you state)
  • width = 68mm (you say 69mm, but 68mm is a standard size, could you be 1mm out?)
  • JIS standard taper (this will make sense to you when you start looking at BBs, the other standard is ISO. In JIS, "J" equals Japan, I suspect in JP400, "JP" also equals Japan)

There's one last variable, unfortunately. This is how the bottom bracket attaches to the frame. It screws in, but there are two different types of thread in common use, English and Italian. The type of threading used depends on the manufacturer of your bike. As a generalisation, Italian threads are (surprisingly) used mostly by Italian makers, the rest of the world pretty much uses English.

So, most likely you'll want an English thread, but you should check this (do you know the manufacturer of the frame?) before buying as the two threads are most definitely not compatible.

NB as always there's a page on Sheldon Brown's website with good information on BBs.

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Thanks, Bike is a Dawes Discovery. Cant tell by google what thread is. But I guess English, as bike is British make. width is probably actually 68mm as I may not have measured very accurately. –  spiderplant0 Apr 16 '13 at 17:51
    
yes if its a Dawes it'll be English. –  PeteH Apr 16 '13 at 18:57
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