Tightening a threadless headset requires tightening the preload bolt (or cap bolt) in the cap on the top of the stem. This bolt is connected to a star nut driven down into the steerer tube that acts as an anchor by gripping the inside of the steerer tube with a downward force. The star nut may be replaced by a self expanding wedge in some designs. The bolt compresses the stem down onto spacers, usually aluminum, which in turn compress the headset bearing cups. The preload bolt does not hold the fork onto the bike; after the preload is set, the stem bolts must be tightened to secure the fork in place. The adjustment must be made such that there is no play in the bearings, but allow the fork to turn smoothly without binding or excessive friction.
In this system, the spacers are important in placing the stem and preload bolt in the correct position on the steerer tube. Thus the stack height of the stem becomes important. The steerer tube of the fork must be cut to length such that it leaves at least enough of the steerer tube protruding above the headset for the stem to clamp on to. Bicycle racers seeking the greatest saddle-to-handlebar drop for better aerodynamics will often forgo spacers and cut the steerer tube down to match the headset bearing cup stack height in addition to the stem stack height.
