Originally Posted by
JohnDThompson
Check the threads on the steer tube. Riding with a loose headset can damage the threads such that the threaded headset parts can no longer hold adjustment.
If you find damage, the fix is either a replacement fork with intact threads on the steer tube, or filling the damaged threads with brass and cutting new threads into the brass.
That's what I was thinking too. I've fixed a similar problem by installing an additional washer under the lock nut so that the locknut can snug up on different, less worn, threads.
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