A rotary drill bit assembly comprising a drill bit body, at least one trunnion formed integrally with the drill bit body and a rotary cutter rotatably supported on the trunnion by bearing means. The bearing means consist of at least two rows of balls which ride between an inner raceway groove formed integrally in the trunnion and an outer raceway groove formed in the interior peripheral surface of the rotary cutter. A filling hole or port is provided in the drill bit body which terminates at its inner end in each raceway so that the balls of the corresponding row may be assembled in the annular space between the trunnion and rotary cutter. A filler pin closes the corresponding filling hole or port and has a terminal end which complements the cross section of the raceway groove.