Disc brake rotors should be regularly checked for wear to ensure safety & performance. The DCA-1 installs onto most digital dial or vernier calipers (including the Park Tool DC-1) & features protruding contact points that allow measurement of the center of the braking surface bypassing the unworn edge to ensure an accurate measurement. The conical end points of the DCA-1 can be used to measure center-to-center distances of bolt holes for calculating bolt circle diameter (BCD) of chainrings & cranks. DCA-1 FEATURES Compatible with most standard vernier dial & digital caliper designs