STAR NUT

Fork star nuts (star-fangled nut). Provides a threaded anchor point that allows you to adjust and "preload" your headset bearings to eliminate steering wobble

When tightening the top cap bolt of the headset, it screws directly into the star nut. As you tighten it, it pulls the fork assembly upward against the frame, setting the correct pressure on the headset

Star nuts are only for forks with metal (steel or aluminum) steerer tubes. Never use a star nut on a carbon fiber fork, as the metal flanges will crack or crush the carbon