Abstract: A boring tool particularly useful for cutting counterbores in an engine block includes means for selectively engaging and disengaging an output shaft which is rotated and axially advanced during the cutting operation. Preferably, the selective engaging and disengaging means includes feed threads formed on the output shaft, and a threaded means which is selectively moveable into and out of threaded engagement with the feed threads on the output shaft. Preferably, the boring tool is of the differential-rate type wherein the output shaft is rotated at a first predetermined rotational rate and the threaded means is rotated at a second different predetermined rotational rate. The differential rotational rate establishes the advancement rate of the output shaft. The position of the output shaft and the cutter element it carries can be readily adjusted without necessity of operating the boring tool to establish the cutter position.