Not really sure there is an answer to this, but its just so frustrating my headset keeps loosening. When it does the bottom race of the upper bearing rocks back n forth in its conical seat. Im afraid it will eventually wear away at the frame making the bearing seat oval. Only solution seem to be loosening the stem, tightening the preload and tightening the stem Again. It just doesnt stay put.

Is this a carbon steer fork or alloy? Have you verified that the compression-plug or star-fangled nut are torqued/seated? This sounds a great deal like a compression plug not torqued...

After completing the cycle of readjusting and tightening the headset and stem remove the top cap and measure down to the end of the steerer. Record and reinstall the cap. Ride the bike till the loosening reoccurs. Remove the cap and remeasure, any change in dimension? Also placing a small paint dot at the edge of the stem and the steerer will be a tell all if the stem moved WRT the steerer. I assume you've applied assembly paste. Andy

Good suggestions so far. However, better information about the type of headset and the specific bike you are dealing with will help immeasurably in providing useful information

Since we're guessing, I'll also guess that its a carbon steerer and the expander plug is slipping. Try carbon assembly paste on both the plug and the stem. My solution to this problem was to install a Deda expander. https://www.universalcycles.com/shop...s.php?id=29439

Thanks,

It is a carbon steerer with what appear to be a long permanently installed expander plug. I haven't noticed any looseness or movement of the plug. I didnt use carbon paste. It didnt come with any and it didnt even enter my mind. Possibly that is the solution.

to be actually Permanent epoxy in a tube of aluminum as a liner sleeve ,

Then you can use a star-fangled nut that is off limits for non metal ie carbon steerer tubes ..

because you put in a metal tube liner for the star to dig into... ...

Some Italian Bike's Forks were intentionally doing that..






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I dont think that is needed. There is already a long plug in there, that is threaded for the top cap bolt.

Is the plug set, AKA torqued as it should be? It shouldn'e just be threaded for the topcap bolt--there should be a hex head nested in there to snug the expander as well.

Not always the case. Some older carbon steerers had a "thin" piece of AL tubing bonded into the top of the steerer. I have installed a very few of these maybe 10-15 years ago. Actually a really great solution until you want to transfer the fork to another bike with a significantly shorter head tube... If done well there's no "plug" slippage at all. Care does need to be taken as once bonded in place there's no removal, the couple I did had a lip that added a couple of MMs to the effective steerer length. Andy.

Not all the manufacturers recommend carbon mounting paste on steerer-stem interface. The grit from the paste finds its way down to the headset bearing and can ruin it. More on that topic:
Upper Headset Bearings Last Forever, Except When They Don?t ? Killa?s Garage

With a constructive discussion in the comment section below, where my eyes were opened.