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My left crank arm keeps coming loose on the square taper BB -I can't tell for sure but whats most likely at fault the arm or the bottom bracket wearing?When I bang it back on and tighten it's fine for a while.I've tried not to ride far when its loose to avoid more damage.

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    Loose crank arms have been dealt with on this site several times before, but TLDR, when a crank get loose the steel axle deforms the tapered hole in the alloy crank arm so it does not fit on the axle properly and keeps working loose again. You may not be tightening the retaining bolt enough, if you get a bike shop to tighten it with a torque wrench it may stay on, but if the taper is too deformed a new crank is needed, unfortunately. – Argenti Apparatus Jun 8 '20 at 19:47
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    Another dirty hack is to use thread locker like locktite in the threads. However this puts some pedalling load onto the threads, and really just gets some more mileage from those parts before replacing. – Criggie Jun 8 '20 at 20:28
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The crank taper surfaces are deformed and the arm needs to be replaced. There is no other fix. Usually the bottom bracket spindle is fine in these cases, but it's not an absolute given and should be looked at. The problem typically originates from the crank being under-torqued initially.

Left replacement cranks are available and not expensive. You need to get the correct length, profile, hole orientation, and color permutation. There are several questions here about this.

  • I've often suspected that one could MacGyver a fix by using a shim cut from the side of a beer/pop can. – Daniel R Hicks Jun 10 '20 at 1:24
  • The bike is an old raleigh mtb and I assume the BB is an open bearing type . if I can get a suitable replacement are the crankset parts a standard fit ie do the square tapers vary? – Keith Bruford Bennett Jun 12 '20 at 19:32
  • @KeithBrufordBennett Unless dealing with antiques, there are two different taper standards, JIS and Campy/ISO, and JIS is ubiquitous except on a few high end brands. – Nathan Knutson Jun 13 '20 at 16:44
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    The one I have now is threaded with seperate nuts – Keith Bruford Bennett Jun 13 '20 at 18:37
  • @KeithBrufordBennett Nutted vs bolted doesn't determine the spindle type and the cranks don't care whether it's a nut or bolt holding them on. 99%-ish or more of square of taper bikes have JIS so it's likely what you've got too unless it's pre-70s or so. I encourage taking this to a well-stocked bike shop if possible because it's a common issue you're working with and selling you a new left crank is pretty trivial for most. – Nathan Knutson Jun 13 '20 at 22:41

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