First you must loosen the stem's steerer clamp bolts. Then tighten the stem's top cap only enough to remove all play when the bike is on the ground with the front brake applied and rocked fore-and-aft. A very light preload (3 to 5 inch-pounds ) may be used if the headset has cartridge bearings. Then center the stem and tighten the steerer clamp bolts to spec.