I have a Shimano UN55 square BB. Within about a month of buying my new bike the Truvativ crank came loose and is essentially too rounded out to salvage. Any suggestions on an alternative crank to go with the existing BB? I would like to avoid replacing the entire crankset if it isn't necessary. Currently running 175mm cranks.

  • 1
    What you've got is a vanilla square taper crank. Any bike shop should be able to supply a replacement, or you can salvage one from a junker. (There is some danger that the crank shaft end has been rounded over -- can't tell without looking.) Dec 3, 2017 at 0:11
  • What exactly are you looking for in an answer? Product recommendations are off-topic and people are voting to close the question on that basis. On the basis of the answer that isn't about particular products, I've voted to leave this open. Dec 4, 2017 at 14:11
  • I do not need a specific product recommendation. The question is more whether I should just replace the left crank which is probably rounded out or upgrade to a non-square BB? It would be nice to only spend $20 to get a new crank arm, but if this is going to keep happening with a square BB even when the crank is properly torqued, then I would just spend the money to replace the entire crankset and BB.
    – Reddpoint
    Dec 4, 2017 at 17:03
  • @Reddpoint - I have ridden square taper cranks since about 1980 and have never had one "round out" on me. Of course I'm careful to make sure the crank fixing bolts are tight whenever I have need to work on the crank, and I shake the crank arms every now and then to check for any hint of looseness, in addition to listening/feeling for problems as I ride. A properly maintained square crank should never give you a problem. Dec 5, 2017 at 3:36

1 Answer 1


When a crankset comes loose, stop riding immediately and tighten it. Even going a little distance can ruin the crankset and possibly the bottom bracket spindle. It sounds like in your case, the crankset is toast.

We don't have enough information to say if you need a new bottom bracket or not (the Shimano BB-UN55/56 are standard square taper (ISO) bottom brackets and cost about 10-15 dollars), but you need to combine that with a square taper crankset. Alternatively, you can replace the bottom bracket to another standard rather than square taper (e.g. Hollowtech II) and get other cranksets with that standard.

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