well when you tighten the screw into the starnut, the top cap pulls the stem down and everything tightens up causing the heaset cone to set. without a topcap it would be really hard to get a headset adjusted just right, almost impossible, because you need that cone to set or your headset will be loose. untighten the sten bolts that clamp the stem to the steertube then tighten the headsetbolt as much as you can before you feel any drag when you turn the bars. then line up your bars and tighten the stem bolts.