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  1. #1
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    ISIS M8 vs M15 bottom bracket nightmare!

    What do M15 or M8 refer to with regards to bottom bracket and chain set. I have an M15 FSA bottom bracket and a M8 SRAM chainset (both ISIS) and they DONT FIT. Very frustrating!

    Seeing as I must change one of them which do you recommend?

    Thanks for the help.

  2. #2
    Senior Member
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    "M" means metric threads which are defined by their diameter in millimeter and thread pitch (the distance between adjacent threads) in the same units. An M8x1.0 bolt is 8 mm in diameter with a thread pitch of 1 mm.

    Most square taper bottom brackets use M8x1.0 mm crank fixing bolts. ISIS and Octalink bottom brackets have larger diameter hollow spindles and most use M15 crank fixing bolts. The two types are not interchangable.

  3. #3
    Senior member Dan Burkhart's Avatar
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    Quote Originally Posted by joswa View Post
    What do M15 or M8 refer to with regards to bottom bracket and chain set. I have an M15 FSA bottom bracket and a M8 SRAM chainset (both ISIS) and they DONT FIT. Very frustrating!

    Seeing as I must change one of them which do you recommend?

    Thanks for the help.
    Not really sure from your description and terminology, but M15 and M8 usually refers to fixing bolt size. If that is the case, the crankset you have is a Powerspline, not ISIS. ISIS BBs take a M15 fixing bolt, Powerspline uses a M8. They are not compatible.
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