Park Tool  DT-1 Disc Mount Facing Tool

In order to ensure disc brakes function smoothly and accurately, the frame’s mounting tabs must be parallel with the brake pads and rotor. The DT-1 is designed to perfectly machine the face of each tab, removing paint and material to create a flat surface.
The DT-1 works on 135mm rear frames with the "standard" 10mm axle dropouts, and 12mm through axles by using a spacer included with the kit. The DT-1 also works on front forks with 100mm axle width and a 9mm axle dropout, and with 20mm through axles by using a spacer included with the kit. For bikes with the 12mm through axle, use the axle from the bike and the bushing included with the DT-1. For 20mm front forks, use the axle from the fork and the bushing included with the DT-1.