Square taper BB keeps coming loose

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  • Square taper BB keeps coming loose
  • When you say “it keeps coming undone”, do you mean that the BB itself comes loose in the frame, or the crank arms are coming loose on the axle?

    If it’s the latter, then I’d suspect that the square hole in the crank armed is borked.

    Premier Icon schmiken

    If it’s the former, try Loctite.

    Premier Icon tmb467

    fitted a new square taper bb to my hack bike only to find the drive side keeps coming undone

    refitted it quite tightly but again, its come loose within a few hundred yards. Got a feeling I did the NDS cup first then fitted DS & bracket…should I do it the other way round?

    any reason why it would happen? Wrong axle length mebbe? all I can think is that the NDS pedal is somehow turning something that then loosens the DS…no idea how tho!

    Premier Icon simon1975

    The driveside threads are presumably the actual body of the cartridge? That needs to be done up fully until it contacts the frame (no need to go mad with high torque, though). Then fit the NDS cup fairly firmly, and not too firmly at all if it’s plastic. In steel frames I lather the threads in copperslip first, not tried loctite there…

    And when it comes to removal, undo the NDS cup first.

    Premier Icon Mr Agreeable

    Sounds like the threads in your BB are on the way out. Next stop Araldite, or one of those threadless BBs that SJS sell.

    Premier Icon tmb467

    its the BB that comes loose in the frame, reckon it might be that I did NDS first then tried to tighten DS/BB up

    dont think there’s a problem with the frame but you never know


    Sounds like the threads in your BB are on the way out.

    That was my next thought. There’s also a VERY slight possibility that the threads on the BB shell and the cups might not be the same (e.g. British/JIS mix or something like that.

    It’s because you tightened up the nds first. You should tighten the drive side first then the nds. It’s the drive side that tightens against the frame the. The nds tightens against the end of the bb shell. So what’s happening is the bb is moving in the frame because its not actually tight. Undo the nds, tighten the drive side and retighten the non drive side. If you no already damaged it (too much) it should be fine.

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