I go into this reluctantly, but necessarily, as I've exhausted all but one option. I picked up a '91 Giant Yukon frameset at the co-op, while in the US last summer. Completely dismantled it for the trip to México. Put it together and rode it some, and then put much better brakes on it, and have had headset problems ever since. It cannot be adjusted, either by me or by my mechanic. By the time i get the slop out of it, it's way tight to be functional. I can get it within reasonable specs, and then when I tighten the lock nut, everything seizes up. It's like the tightening of the lock nut is compressing the head tube, and I just can't imagine this being the case. I have a mechanical background, having worked on motorcycles, cars, trucks, tractors and farm implements, and can usually find my way through. I've tried a different headset, with the same result. The one option I haven't exhausted is trying a different fork. Once i put a good front brake on it, I noticed the slop, which I didn't notice before.
Thoughts?

Lot's of possibilities. First is that no one has adjusted the headset well. (Got to get that one out of the way). Steerer is bent. Crown race is not well faced. Head tube not faced. One of the cups/races are ovalized. Wrong size or number of balls. Retainer rings in up side down. Steerer theading is worn and threaded race jumps a thread on tightening.

The reference to better brakes makes me wonder if there was a problem all along but you never noticed it until there was more brake grab. Andy.

Thanks Andy,
I suspect the steerer tube, but won't be able to get to it until mid December, when I return from the US. The rest I well try in the next few days.

Please don't take this the wrong way, but you don't know the history of this bike and have taken the the cheapest possible route to owning a mtn bike. So, just ride it. The bike probably doesn't have many miles left in it so your time (and money!) is better spent elsewhere.

You don't say if the headset ever worked properly or if the headset has never worked or the headset was removed and re-installed. There is the possibility the fork crown was left off or installed upside down. A friend years ago asked me to fix this very problem after installing a fork on a new frame; he didn't install the crown race.

I suspect that the steerer tube and/or stem adjuster nut threads are stripped to some degree. This would explain why "everything locks up" when you tighten the lock nut.

This is not an uncommon problem with headsets that use slotted washers but can also occur with older dual nut style. Just because you have set the pre-set with the threaded cone, does not mean it will stay that way when you tighten the lock nut; tightening the lock nut will increase the pre-set and lock-up the bearings. The solution is to loosen the threaded cone a little and try it again; if it's still too tight loosen the cone again. The threaded cone may seem too loose, but eventually you'll find that spot where the threaded cone is adjusted so when the lock nut is tightened, it will pre-set the bearings without locking them up..

Step 1: Inspect the frame/headtube and under the downtube near the headtube. If rippled or paint bulging, then most likely, you have a bent frame. Might not be salvageable. Might. But may cost to chase/face and realign.

Step 2: Loosen stem/handlebars, headset assembly. Inspect caged bearings, upper and lower cups, crown race, seals. Make sure the bearings are oriented correctly. Make sure cups and crowns are not pitted. Make sure there aren't dust seals that have been pinched between cups and crown. Make sure the crown race and cups are seated flush against fork/frame respectively. make sure the bearings are the right size and stack order. Many headsets these days will have slightly bigger bearings on the lower cups/crown. If you have a more bulbous lower cup, that's probably a sign. Inspect lower crown race - make sure it exists and hasn't fallen and disappeared, or it isn't fractured. Many lower end only steel forks use a 27.0mm crown race ID. The higher quality forks take a 26.4mm. Most decent replacement headsets are 26.4. If the mechanic tries to force a 26.4mm on a 27.0mm fork, well, welcome to micro-fracture city and you'll need to replace the crown race with something compatible that's 27.0. It's important to measure -accurately- such headset dimenions on older forks.

3. Try to re-assemble and adjust.

4. If that doesn't work, you may want to try a economical Tange headset. They cost $12-$15 and work well and last for a while.

I assume that when you say you 'completely dismantled it', you don't mean that you removed the cups and fork race. Starting from there and assuming that the headset had caged bearings (most do), I'd suggest you look at the orientation of the bearings. They have to be put in a specific way and if you get them in upside down, they will bind.

On the other hand, if the problem can't be resolved, a new headset could fix all of your problems. They are cheap or expensive depending on the flavor. I'd go with a cheap one.

Is there some reason you (OP) not responded to my posts? If you have already tried my recommendations and they did not work I'd like to hear about it, so if there is another solution I can learn from it.

BTW, you can pick up a new headset for your bike for less than $20 (sometimes less than $5), you just need to know if it is ISO or JIS..https://www.amazon.com/s/ref=sr_nr_sc...6c09fd50133d2a

I haven't replied, because I've been cruising Classic and Vintage. History: I walked into the co-op looking for a mtn bike, as this one was in the stand having its components transferred to a Cannondale frame. It didn't occur to me to ask why. Next time I'll ask. I have already replaced the headset, with a new, cheap one, and have the same issue. Sometime tomorrow(or maybe this evening), I'll try loosening the threaded cone; this possible solution makes sense. And, if I find the time, I'll try all the other suggestions. I leave for the US early Tuesday morning. Mainly, I asked for help because I couldn't understand what was going on, after weeks of thinking and trying different things. If I can understand it, I can figure out how to make it work, usually. I'll get back to you in the near future.
I took the retainers out and went with loose bearings, at some point, just because I'm a loose bearing kind of guy, and the retainers I can find here cost about a nickel--figured they couldn't be very high quality. Maybe I'm wrong here. Maybe I put too many loose bearings in. What I'm finding, particularly with the bottom race, is that when I remove the fork, there isn't any grease on the pressed on race. I'm also the kind of guy who uses lots of grease, so this has me concerned that maybe the bearings I used are too small. Anyway, enough prattling. I'll go try the loosened cone trick.
Cyccocomute: I didn't remove the headset for bringing it down, but I did do it when I put the new headset on, per instructions I found on Sheldon's site.
FMB42: I replaced the headset because the threads in the locknut were definitely compromised. The threads on the steerer tube appear to be fine, but forks are cheap here, and if I don't find another solution, I'll swap in a new one when I get back.
Cale: Could be, but I still want to learn how to adjust a headset.
gyozadude: I've checked that already, but will do it again, more closely. I've looked at the fork a lot, and can't see a bend, but that's not to say there isn't one. I'll definitely try bigger bearings in the lower cup. The new crown race went on more easily than I expected, so is likely the correct size. I wouldn't call it a quality headset, per se. But new nonetheless.
I'll get back to you all tomorrow.
Seeds

Ok folks,
I went home last night and tried some things while it was still fresh. Remembered that the new headset I bought had an adjustable cone with no threads, so I kicked it out and put the old one back on. Just to be safe, I tried it with that one and, of course, it couldn't be adjusted. This morning I took the whole thing apart. Imagine my surprise when I found not one, but two retainers, both installed with the closed side toward the pressed in cup. I also found a fiber ring, probably a lower seal, inside the adjustable cup. After removing that, I got much better results, but still not what I'd like. I kept fiddling and eventually the locknut stripped. So I'll shelve this project and return from the US with a decent headset. I read in Sutherlands, this morning, that some headsets just aren't meant to work.
But, thanks to you all, I now feel I have a reasonable understanding of the situation and look forward to better results next time. I'll report back in a month or so.
Seeds
PS I'm convinced the frame is neither bent nor buckled.