I'm at a loss with the headset on my 10+ year old mountain bike. I looked at previous posts but never really found an answer. The fork moves front and back but I know it's not the brakes. I can't tighten the star nut any farther. The bearings are original to the bike but I don't see any issues like warping or loose bearings. I didn't remove the headset but I didn't repress with no change. Im wondering if its the plastic ring that the spacers sit on and wedge onto the fork post.

On a threadless headset, the star nut and the bolt that goes into it through your top cap
do not actually hold your bearing adjustment. you need to loosen the machine screws that
tighten the sten onto the fork tube, then reset the bearing preload using the top cap bolt,
then retighten the stem in its new position.

It's also possible to damage or lose a spacer. They need to go up a little farther than the
end of the steerer tube in order to set that preload.

+1,
The headset is pobably fine, except that it might want to be cleaned and relubed, but that's not causing the problem.

Either you're missing a spacer and the top cap is bottoming out against the top of the steerer - see sketch, note gap at top of steerer tube.

Or it's even simpler and you're forgetting to loosen the stem bolts before adjusting. The sem won't move unless you loosen it.

One other possibility (rarer) is that prior efforts to force the adjustment have jacked up the star nut, so the top cap screw is running out of thread.

Either way, those are the headset related issues.

Lastly, on an older mtn bike, it's possible that the headtube diameter got stretched out over the years, and the lower cup is loose in the frame. This is most likely if the bike has seen hard service, and the lower end of the headtube isn't reinforced with a band. It's a very easy diagnostic to confirm or clear his. With the front brake on, rock the bike forward and back while holding your fingers where the lower cup meets the headtube. If the cup is moving you'll definitely feel it.

Fixing this is a bit complex, but it is doable. If you confirm that the cup definitely is loose, see a local dealer, or come back and we'll take it from there.

you mention the plastic ring...it may be binding and not allowing you to preload the adjustment. Try sliding the ring and spacers off and on to make sure it is mobile. What kind of fork? Movement can come from worn fork bushings if it is a suspension fork, may not be headset.

Fork is new (Surly rigid) and there wasn't any problem with it before swapping to this bike. I did forget to mention that my fork is uncut and I just use extra spacers above the stem. Again, no issues with the spacer set up on the other bike. The plastic ring does not sit flush in the cap, which seems iffy to me.

Maybe post a photo of the plastic ring area. I'm not sure what you are referring to.

Are the spacers above the stem higher than the fork? This is critical to make the headset function correctly. With the two stem bolts loosened, the top cap "pulls up" on the fork steerer tube, making the stem and spacers press down on the headset bearing.

The spacers should be higher than the steerer tube, just like the stem is in this photo from the Park Tool Headset page.

Yeah the last spacer is above the fork, I know about that part, so the cap has something to press down on around the tube. Ill get a pic this afternoon, thanks.

Forgot I took this pic that shows the plastic ring not flush. Look above the area being pointed out, the spacer is sitting on the plastic ring (not the cap). I'm not sure how else to get that ring clamped down.
*Edit, not current set up but still shows current issue

I've got a very similar issue. I didn't want to bury it inside your post so I posted it here. I'd appreciate your insight. Thanks in advance.

https://www.bikeforums.net/showthread...7#post15999527

It's ok maybe we can solve both! This is another pic of the exact set up on my red bike same fork, spacers, cap and ring. See how the ring doesn't sit inside the cap?
*Sorry the pointer isn't on the current issue, see above it.

If the wedge ring doesn't sit at least a little proud of the bearing cone, the spacer can't bear down on it and make the cone tight on the steerer.

I'm not coming up with much wrong, looking at your pic - that looks like it should work.

Try holding the front brake on, and rocking the bike back and forward, while you try to narrow it down. Use your fingers as well as your eyes.

Most threadless headsets have a conical split ring installed above the upper bearing cartridge and below the top cap to keep the steerer centered. The cone's narrow end must be installed downward and fit inside the inner race of the bearing. Any chance that split ring is installed upside down? That would keep the upper cap from seating fully and keep the headset from adjusting properly.

It's in there as you described but it sure doesn't want to stay in there flush. I'm wondering, I didn't see a split/wedge ring on the lower race though it does have a plastic ring that sits on the race.

The split wedge ring is only on the upper bearing. There is none on the lower one.

Ok good, you guys have exhausted your time, thanks. Ill try a few more things this eve otherwise ill take the top cap and wedge ring to the LBS and maybe see what's up.

Caged Bearings upside down?