Thanks so much for your response. There is a star nut hammered into the steering tube, so I doubt I'm pulling that up. It must be as you say that I'm bottoming out the headset cap on the steering tube. But I wonder, then, why the bolt keeps turning. Am I just flexing the cap (which does appear to keep moving down)? If so, do I need to replace the cap (that is, have I ruined it) when I add a spacer and try again? Also, inasmuch as the movement is so slight, what am I risking by just ignoring the problem? Thanks again.