Originally Posted by
mazdaspeed
If you don't have a spacer on top it's easy to bottom out the top cap on the steerer tube and not be able to preload the bearings enough. On cane creek integrated headsets the spacers have an inner circular protrusion (they also 'click' together when you use them in a stack) that fits into the top part of the headset, it increases the contact area considerably compared to just slamming the stem onto the top of the headset.
you can fix that just by adjusting the expander plug further down into the steerer tube.
i had that happen on one of my bikes simply because i switched to a stem that had a shorter stack height - so with the same number of spacers, the top cap wasn't going down far enough to preload the headset.
i just loosened the expander a bit, set it down another 10mm, re-tightened it.