Abstract: A drill stem has a drill bit for use in drilling a bore hole in an earthen formation, this drill bit being on the down hole end thereof. An eccentric mass rotor is mounted in the stem spaced between the opposite ends thereof. The rotation axis of the rotor is oriented such that when the rotor is rotatably driven a rotating force vector component is generated, this force vector lying in a plane substantially normal to the longitudinal axis of the bore hole. The rotating force vector causes a portion of the drill and stem assembly to precess or roll around against the wall of the bore hole in a cycloidal manner and in forceful engagement with the bore hole wall to thereby cause rotation of said drill stem about its own axis either to directly effect drilling action or aid drilling principally achieved by some other drive means coupled to the drill stem.