Park Tool DCA-1 Digital Caliper Accessory. Disc brake rotors should be regularly checked for wear to ensure safety and performance. The DCA-1 installs onto most digital dial or vernier calipers (including the Park Tool DC-1) and 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 and cranks.Compatible with most standard vernier dial and digital caliper designs