Originally Posted by
Rowan
If you have the steerer tube protruding, the cap won't be able to pull down on anything except the tube, leaving you with loose and very clunky stem and headset bearings.
You just add one or more spacers over the top of the stem. Put enough spacers on so that the top of the steerer tube is below the top spacer. The cap then goes over the top and will snug down to preload the headset bearings.