An airhammer embodies an outer housing structure connectible to a rotatable drill pipe string through which compressed air is conducted. A hammer piston reciprocates in the housing structure, compressed air being directed alternately to the upper and lower ends of the piston to effect its reciprocation in the structure, each downward stroke inflicting an impact blow upon the anvil portion of an anvil bit extending upwardly within the lower portion of the housing structure. Upon elevating the airhammer off the bottom of the bore hole being drilled, the anvil bit drops downwardly of the housing structure to open one or more air bleeding grooves in the anvil bit, compressed air in the housing structure bleeding quickly through such grooves and preventing or stopping the hammer piston from reciprocating and impacting against the anvil bit. A step is also placed in the bit at the junction between its head and shank to break the stress wave, incident to the hammer piston striking the anvil bit, before the wave reaches the bit head, thereby reducing the induced stress at the head and shank junction, which increases considerably the fatigue life of the bit, thereby preventing breaking of the shank at its junction with the bit head.