that the shank may be closely received in receptacle 18 shank 46 therefore includes an upper abutment surface 48 dimensioned for receptive engagement with surface 24 and generally planar rectangular surfaces 50 52 and 54 which are dimensioned to be closely accommodated between containment walls 26 28 and 30 respectively each of opposing surfaces 50 and 52 are traversed by grooves 56 and 58 respectively which grooves are dimensioned and located to be complementary to corresponding grooves 32 and 34 and alignable therewith so that when the shank is received in receptacle 18 groove 32 closely aligns with groove 56 and groove 34 closely aligns with groove 58 to form a pair of generally cylindrical parallel slots a transverse passageway 62 extends generally transversely through the shank a fluid passageway 64 obliquely communicates with transverse passageway 62 and leads to the bearing assembly the passageways function to conduct compressed air to the bearing assembly to provide a means for freeing the bearings from particulate matter generated by the cutting action of the roller cutters the drill bit unit 14 is rigidly locked to the drill head by means of a pair of opposing lock pins 66 which are received in the.