Abstract: A drill having plural forwardly projecting circumferentially spaced cutting bits includes a hydraulic motor operated at full drilling fluid pressure to drive the cutting bits in unison through a first one-way clutch in a drilling mode. A centrifugal device responding to higher speed rotation of the drill operates a reversing valve connected with the hydraulic motor to reverse it at required times to incrementally advance the cutting bits of the drill in unison to compensate for wear and present a new sharp cutting face on each bit for continued drilling. The reversal of rotation of the bits is transmitted from the hydraulic motor through a second one-way clutch of the single revolution type which drives a pinion common to and in mesh with drive gears individual to the several cutting bits and matching the shapes of their flutes and being axially slidable relative thereto.