Abstract: The rotary drilling head assembly of the present invention includes a housing having a bore for receiving a drive member. The drive member has an outer diameter of less than 17 ½ inches to pass through a 17½ inch opening in a rotary table. A bearing assembly is disposed between the housing and drive member allowing the drive member to rotate within the housing and includes an outer stationary portion and an inner rotating portion. Retaining clamps attach the outer stationary portion to the housing and the rotating portion to the drive member. Rotary seal assemblies isolate the bearing assembly and its lubrication system from the drilling fluid and prevent premature wear and failure of the bearing. The bearing assembly includes a plurality of opposing disc-like members that have flat bearing surfaces meeting on a substantially planar surface of contact. The disc-like members are preferably made of a polycrystalline diamond material.