I've have had an issue with the threaded headset on a mid 80's Specialized Stumpjumper that I would like some help on. This bike was made prior to suspension forks, so it's a rigid front end.

I had an alloy headset on the bike and an alloy cable stop held by the headset nut. I couldn't keep the nut tight; on every very bumpy ride off road, the nut would loosen. (When I say bumpy, I'm referring to trails that are mainly chunky broken rock that makes your arms tingle from the shaking.) Finally, the nut stripped when I was riding.

Figuring that the alloy cable stop was not allowing enough nut thread engagement, I switched the front brake from cantilever to a linear pull (V) brake; this eliminated the alloy cable hanger. I did this change when I changed out the headset for a really old, but new in the box, Shimano 600EX. On the first ride after installing the new headset, it was the same loose nut problem.

When I installed the new headset and got it adjusted, I noticed that if I turned the fork 180*, it would go from just right to very tight in the bearings. The fixed cups are installed fully and square with the frame, so that's not an issue. I'm wondering if the head tube surfaces aren't square or if the fork steerer tube is bent.

Any ideas what is causing this problem?