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The crank arm on my bike fell off. How does one re-attach it?

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  • Do you know why it fell off? Adding that might help.
    – user2525
    Commented Sep 3, 2016 at 17:52
  • @user2525 I have no clue why. I was about to take my bike out yesterday for a ride and when I tarted peddling it was very wobbly and I just took it off with no force.
    – Okiic.
    Commented Sep 3, 2016 at 18:08
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    FYI, your crank arm is probably toast. If you reattach it it will probably be ok for a short while but it will loosen again.
    – whatsisname
    Commented Sep 3, 2016 at 20:32
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    Basically you need to cycle backwards about 500 miles, to when you first noticed the crank arm making noise and feeling loose, then remove the dust cover and tighten the nut.
    – Daniel R Hicks
    Commented Sep 4, 2016 at 2:07

3 Answers 3

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The first two threads on the spindle look stripped, meaning the nut will have stripped threads as well. The square taper in the crank is almost certain damaged from being used while loose. In other words, in my local slang, "it's munted"

The proper, safe and reliable fix probably requires a new BB spindle and almost certainly a replacement crank.

If you do not want to spend that much (I don't recommend this, as the crank coming off is a safety concern), replace the nut - its cheap. If you can source an used crank it should only cost a few dollars (local bike cop, recycle center or rubbish tip) I would do that rather than reuse the one that has fallen off.

As already said, the nut and a washer will be under the dust cover on the crank.

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The picture appears to show a cover over the access hole in the crank arm. Remove the cover by either unscrewing it of prying off the crank arm. Inside you should find a flanged nut. Wipe any grease or grit off the shaft Install the crank arm in the correct orientation onto the shaft. Tighten the nut with a socket (usually a 14mm) to between 300 and 400 inch pounds. Reinstall the dust cover.

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There should be a nut and washer at the threaded part of the axle. Get new ones, put on crank, washer and nut and tighten.

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  • And do you feel there might be any issues with just reattaching a part that failed in such an unusual way?
    – David Richerby
    Commented Sep 5, 2016 at 9:28
  • I'm an engineer. I don't feel.
    – ojs
    Commented Sep 6, 2016 at 16:39

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