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I got an used bike with a worn out left crank arm. I tried replacing with a new square taper crank arm and torqued it to 40nm; however, it started creaking soon after and the bolt had become loose (I checked with a torque wrench).

Could the bottom bracket spindle be worn out as well? I did notice a very tiny bit of wiggle when I fitted the new crank arm before tightening the bolt.

Edit: the original worn crank was steel.

The square spindle (the best pic that I took): bb

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    Certainly possible. Can you post a clean and well lit picture of the spindle's square taper? – Criggie May 21 at 10:01
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    You may have a mismatch between JIS and ISO standard. While JIS is the most common you may have an ISO standard crank axle. They are difficult to tell apart visually. If you can post pictures and tell us the brand and model we may be able to make that determination. – mikes May 21 at 10:12
  • Is this the same bike as in bicycles.stackexchange.com/questions/59590 – Criggie May 21 at 10:15
  • @Criggie Yes. I thought I'd try replacing only the crank arm after that question. – Sunny yue May 21 at 10:21
  • @mikes I'll check the standard by measuring tomorrow. – Sunny yue May 21 at 10:39
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The spindle looks a little rusty. That may be what is causing the the creak. Try completely removing the rust with a wire brush and thoroughly cleaning the crank and spindle before re-installing.

Be aware that the bolt can loosen slightly in use as the crank 'climbs' onto the square taper. See https://www.sheldonbrown.com/brandt/installing-cranks.html

  • I know that it'll loosen from normal usage. The problem is that the creak becomes audible 6 minutes after tightening. – Sunny yue May 22 at 3:51
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I found it to be indeed as a result of the worn spindle after removing the arm and inspecting. I'm replacing the BB as suggested by @Criggie.

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