Headset spacer hate

It's the steerer tube that matters. Most carbon fork manufacturers now mandate a spacer on top of the stem. In my experience, stems can also walk up aluminium and steel steerers if the top of the stem is above the top of the steerer, so I recommend a spacer on top for all steerer materials just to maintain headset adjustment, but it's safe (for the integrity of the steerer) to have the top of a metal steerer a couple of millimetres below the top of the stem.

Oh man, if only someone made some sort of top cap that would remove the need for an ugly spacer on top of the stem. Never going to happen though...