Quote:
Originally Posted by Mark McM
I think you mean the expander plug (expands to grip the inside of the steerer tube).
If the headset won't stay in adjustment, it is doubtful that it is the expander plug. The job of the expander plug/top cap is for setting the initial headset pre-load. Once the pre-load is set, the headset is locked in place by the tightening the stem on the steerer tube. After the stem is tightened, the headset should stay locked into adjustment even if you remove the expander plug. (Note: Some carbon fork manufacturers recommend leaving the expander plug in place to help reinforce the steerer.)
As mentioned, one possibility is that the crown race or cups haven't completely seated. Or, the stem may not be properly tightened on the steerer. Or, the bearing race seats may not be cut square (or the bottom of the stem clamp may not be square). Or, it is even possible that the steerer itself is cracked/damaged. In any case, you should get the to bottom of what is causing the headset to loosen.
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I'll add one other point that may come into play. Sometimes the top cap can come in contact with the top of steerer tube. Make sure there is enough space between the two.