A pretty simple task, installing a headset. Get the pieces on the tube and in the fork. Then install your spacers, stem, maybe a cable guide and you're set. But there's a foolish mistake, that is easy to make. Needless to say, I know from experience.
In this picture you see my recently built up bike, with a potpourri of spacers filling up the uncut fork. Look closely at the gap between the top of the spacer and the top of the tube, it is very thin.
When the cap of the headset is tightened down, no matter how tight you make it, the headset will not come together properly, and when you set out for a test ride, the fork will rattle and the tube. Not really the desired result.
Here I've put a thicker spacer on top, and notice the gap is larger. Now the cap will tighten down on the spacer and keep the whole thing together properly.
Any shop mechanic will give you an "of course" look when presented with this tidbit, but for the home enthusiast, I had yet to see this clearly presented in any book.
