The Problem Solvers Micro-Adjustable Headset Spacer goes on the fork (it requires 10mm of clearance on the steerer) and locks in place to retain your headset bearing adjustment. This makes it possible to run your fork and headset without a star nut or compression plug. This 2-part threaded component is made of CNC-machined aluminum with a small bolt for tightening, so it's light and durable.