Summary - I'm using an FSA Orbit MX threadless/integrated headset in a brand new steel frame. My problem is that I cannot seem to adjust the headset to the right tightness - either it is too tight and the forks do not turn freely, or it is too loose and there is substantial play/knocking.
To elaborate on the problem - I'm building up a new bike from scratch. Rather than try to install the headset myself with homemade tools I took it to a mechanic last weekend to have it professionally installed.
After building the bike up I noticed that the headset was quite loose (a lot of play when tightening the front brake and rocking the bike forward/backward).
So, I tightened it by loosening the stem bolts and tightening the top bolt. That made the play go away, but the fork would no longer swing freely. Loosened a little, and the play returned. I tried for an hour but could not find a middle ground. Either it's too loose or too tight, I can't get it right. Removing the spacers didn't help. The star nut seems to be located properly, a few mm below the top cap. I removed the fork completely and reassembled the upper and lower bearing races and seals, ensuring they are properly seated. No luck.
It's a mystery. Before I bring it back to the mechanic - is there anything I'm missing?
Edit - some photos of the headset, as requested. Sadly I don't have enough reputation to post more than 2