Near the bottom of the Park Tool instructions (
http://www.parktool.com/repair_help/...readless.shtml ) it shows the gap between the top of the steerer tube and the top of the stem as being a couple of mm. The plug is supposed to go into the steerer tube and not on top of it, otherwise you wont be able to pull the stem down onto the bearings. Get a thinner spacer, so you can get the stem clamping the steerer over most of the height of the stem.