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After years of use and multiple retightenings my crank will fall off my bike about once a week and definitely needs to be replaced. I believe that the square hole of the crank has expanded to the point where it can no longer get a tight grab on the bottom bracket

My question is whether I should replace the bottom bracket when I replace the crank given that it's bearings are still good. Is it possible that BB spindle deformity is also part of the reason the crank is falling or is it probable the spindle is fine and I just need to replace the crank.

For reference, the bb spindle looks fine .

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  • You should have replaced the crank arm long before it got bad enough to fall off. By now it's likely that the BB is shot.
    – Daniel R Hicks
    Commented Apr 14, 2015 at 0:16
  • (However, you're in a good position to experiment with something I've considered: Cutting shims of beer can aluminum to tighten up the crank. If you observe the side of the beer can you will see that the thickness tapers in the vertical dimension, and shims cut from a can might be fitted in a way to restore the tight fit between crank arm and shaft. Or it might not work -- who knows?)
    – Daniel R Hicks
    Commented Apr 14, 2015 at 0:19
  • @DanielRHicks: I tried doing that twice several years back, one situation was a worn crank, and the other was to resolve an ISO/JIS crank/BB mismatch. In both situations the beer can shims didn't work very well.
    – whatsisname
    Commented Apr 14, 2015 at 3:26
  • @whatsisname - OK, good to know.
    – Daniel R Hicks
    Commented Apr 14, 2015 at 12:00

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Without seeing any pictures, I think you'll likely want to replace both, but you can figure it out easily enough.

Usually, the bottom bracket will be fine after a crank improperly coming loose. The bottom bracket spindle is typically made out of a hard steel and only the most miserable metal cranks are made from anything other than aluminum, so when the crank gets loose, the aluminum gets wrecked and the steel axle stays ok.

However it sounds like you've caused this to happen several times, so you may have managed to round the axle as well.

If you have had many years of use of this bike so far, I think it would be worth it to just cough up 15-25 bucks for a new square taper BB in addition to a new crank and not have to think about it anymore. Your existing BB might be getting worn out already and so you can kill two birds with one stone. You can read the Park Tool guide to cartridge BB servicing. If the bearings are getting worn with age, then it makes this whole ordeal a no-brainer.

Otherwise, if you put your new cranks on your existing bottom bracket, and you pay very close attention to whether it's getting loose or not, you'd probably be fine in determining if the existing bottom is fine, and if it does get a little bit loose, replace the BB immediately and your new cranks should be fine. Do be careful because if they get so loose they come off, you could possibly damage your new cranks.

Ultimately it boils down to whether to pay 15-25 bucks up front, or gamble the price of your replacement cranks that they won't come loose. Only you can decide that.

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  • Yeah, I think I agree that I should probably just get new BB given that it's pretty cheap. It will probably need replaced in a year or two anyway. thanks!
    – Andrew
    Commented Apr 13, 2015 at 21:36

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