A down the hole drilling assembly having a top end arranged for coupling to a drill string and bottom cutting end, the drilling assembly comprising: an elongate casing having an outer wall and an inner wall; a bore housed within the inner wall of the casing having an inner bore diameter D 1; a fluid powered piston arranged moveably inside the casing which is capable of shuttling axially back and forth, the piston having a central portion with a cross-sectional diameter D 2, a top end distal portion with a cross-sectional diameter D 3 and a bottom end distal portion with a cross-sectional diameter D 4; a top working chamber arranged at the top end of the piston; a bottom working chamber arranged at the bottom end of the piston; a top control sleeve and bottom control sleeve arranged inside the casing; a plurality of fluid passages located between the control sleeves (20,60) and the casing including: at least one main feed passage, at least one top feed passage and at least one bottom feed passage arranged to control the feeding of pressurized fluid into the top and bottom working chambers to generate the reciprocating movement of the piston; at least one flushing port at the bottom end of the casing which is connected to at least one bottom vent passage arranged to exhaust the bottom chamber; an exhaust system comprising at least one exhaust port and at least one exhaust passage at the top end of the casing arranged to exhaust the top chamber via at least one top vent passage; and an air distributor having at least a first fluid passage connecting an inlet port to the at least one main feed passage and a second fluid passage connecting the top vent passage with the at least one exhaust passage; characterized in: the piston having a top work area (Wi) and a top intermediate work area (W 2), wherein the cross-sectional area of the casing bore (A CB), is equal to the sum of the top work area (Wi) and the top intermediate work area (W 2): W 1 + W 2 ⩾ 0.99 * A CB an elongate casing having an outer wall and an inner wall; a bore housed within the inner wall of the casing having an inner bore diameter D 1; a fluid powered piston arranged moveably inside the casing which is capable of shuttling axially back and forth, the piston having a central portion with a cross-sectional diameter D 2, a top end distal portion with a cross-sectional diameter D 3 and a bottom end distal portion with a cross-sectional diameter D 4; a top working chamber arranged at the top end of the piston; a bottom working chamber arranged at the bottom end of the piston; a top control sleeve and bottom control sleeve arranged inside the casing; a plurality of fluid passages located between the control sleeves (20,60) and the casing including: at least one main feed passage, at least one top feed passage and at least one bottom feed passage arranged to control the feeding of pressurized fluid into the top and bottom working chambers to generate the reciprocating movement of the piston; at least one flushing port at the bottom end of the casing which is connected to at least one bottom vent passage arranged to exhaust the bottom chamber; an exhaust system comprising at least one exhaust port and at least one exhaust passage at the top end of the casing arranged to exhaust the top chamber via at least one top vent passage; and an air distributor having at least a first fluid passage connecting an inlet port to the at least one main feed passage and a second fluid passage connecting the top vent passage with the at least one exhaust passage; characterized in: the piston having a top work area (Wi) and a top intermediate work area (W 2), wherein the cross-sectional area of the casing bore (A CB), is equal to the sum of the top work area (Wi) and the top intermediate work area (W 2): W 1 + W 2 ⩾ 0.99 * A CB