Typically a pinion has a number etched on the gear face indicating the correct depth between it and centerline of the ring gear. The pinion depth checker is essentially a dial indicator stand that registers from the carrier bearing bores. The purpose of this tool is to shim the pinion to it's specified depth which expidites setting up the pattern between r&p. Keep in mind the r&p is burnished as a pair at the factory which is where the pinion depth figure comes from.
