Abstract: There is provided drilling apparatus (10) comprising a hammer casing (11), a free piston motor piston guide and porting sleeve (12), a drive sub (13), a drill bit (15) splined by shank (16) to the drive sub (13). Below the splined portion (17) the drive sub is relieved at (23) to form an annular air space for motor exhaust air from the splines (16) and (17). The drive sub (13) retains a gauge sleeve (24) via an annular flange (25) between the hammer casing (11) at (26) and a shoulder (27) on the drive sub (13). Below the shoulder (27) the outer diameter of the drive sub (13) is provided with eight grooves that form conduits (30) with the lower portion (31) of the gauge sleeve (24) extending parallel to the drill axis in communication with the space (23) via ports (32). The lower portion (31) extends down the side of the drive sub (13) and terminates with it at (34) adjacent the shoulder (33) at the transition from the bit shank (16) and the bit head (35).