The lower nut does the adjustment. Tighten so there's no slack/still swivels freely. The top nut then clamps down to lock the lower one in position. You'll need 2 spanners - some headsets just have a knurled edge (use grippers -carefully), some are smooth with holes for specialist locknut spanners (maybe take to your LBS).
The lower nut does the adjustment. Tighten so there's no slack/still swivels freely. The top nut then clamps down to lock the lower one in position. You'll need 2 spanners - some headsets just have a knurled edge (use grippers -carefully), some are smooth with holes for specialist locknut spanners (maybe take to your LBS).