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It's a second hand bicycle that I've owned for a short time, but the problems seem to have started after a crash. The headset started to rattle, and I didn't have a tool to tighten it, so I would just do it with my fingers every 30 minutes of riding. Now that I have a tool, when tightening the locknut, it "overturns", i.e. you can't tighten it.

I'm guessing that it's either the threads on the locknut that are worn-out or the threaded tube on which the locknut screws on. I'm not sure how to identify which though. If it's the threaded tube, is that easy to replace, or do you have to change the whole fork?

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Well if both nut and the lock nut are skipping then chances are that thread on fork is stripped. Maybe it would be possible to repair threads on the fork, but since you have been in the accident it would be vise to get your fork and headset examined, maybe there are some other issues with it now.

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You'll need to take the locknut and top race off to examine the threads on the race, locknut and steerer.

What you are looking for is the 'peaks' of the threads being flattened off. Using a bright light helps. If the locknut just spins on the steerer without tightening the damage should be obvious.

If the steerer threads are damaged you'll need a new fork.

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You have received two good, detailed suggestions already, but I would definitely recommend that you have a good read of this checklist if you haven't already.

When you take the setup apart you may find that it's obvious that a particular part is damaged (like it was in my case a few years ago - picture below) but bear in mind that there may be some less visible damage to other parts too.

broken stem

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