A pneumatic percussion hammer having: a substantially tubular casing having upper and lower ends, defining a longitudinal axis (α); an actuating piston having upper and lower ends and supported within the casing for reciprocal motion along the axis; a bit having an upper end supported within the casing and confronting the lower end of the piston and a lower end extending from the lower end of the casing; a back air chamber in the casing above the piston and a front air chamber in the casing between the lower end of the piston and the upper end of the bit; a pneumatic air supply inlet and associated passages and porting, to alternatively impose a high pneumatic drive pressure in the back air chamber against the upper end of the piston, thereby driving the piston downwardly in an actuation phase into impact on the bit, followed by a high pneumatic pressure in the front air chamber against the lower end of the piston, thereby separating the piston from the bit in a retraction phase; wherein said passages and porting comprise: a tubular air feed passage extending into the piston; a feed port associated with the air feed passage in the piston and remaining within the piston as the piston cycles between the actuation and retraction phases; air delivery passages (32, 34, 36) alignable between the feed port and the front air chamber; and wherein the air feed passage is a tube having a closed end mounted for relative axial movement within the piston; and the feed port is defined by at least one aperture in the tube wall adjacent the closed end; Characterized in a valve for the feed port in the form of a sleeve slidable around the air feed passage between back and front limit positions within the piston; whereby while the piston is advancing toward the bit during the actuation phase the sleeve is at the back limit position, closing said feed port, and when the piston impacts the bit said sleeve slides to the front limit position, opening said feed port and thereby delivering pneumatic pressure from the air feed passage through the air delivery passages (32, 34, 36) to the front air chamber to initiate the retraction phase. a substantially tubular casing having upper and lower ends, defining a longitudinal axis (α); an actuating piston having upper and lower ends and supported within the casing for reciprocal motion along the axis; a bit having an upper end supported within the casing and confronting the lower end of the piston and a lower end extending from the lower end of the casing; a back air chamber in the casing above the piston and a front air chamber in the casing between the lower end of the piston and the upper end of the bit; a pneumatic air supply inlet and associated passages and porting, to alternatively impose a high pneumatic drive pressure in the back air chamber against the upper end of the piston, thereby driving the piston downwardly in an actuation phase into impact on the bit, followed by a high pneumatic pressure in the front air chamber against the lower end of the piston, thereby separating the piston from the bit in a retraction phase; wherein said passages and porting comprise: a tubular air feed passage extending into the piston; a feed port associated with the air feed passage in the piston and remaining within the piston as the piston cycles between the actuation and retraction phases; air delivery passages (32, 34, 36) alignable between the feed port and the front air chamber; and a tubular air feed passage extending into the piston; a feed port associated with the air feed passage in the piston and remaining within the piston as the piston cycles between the actuation and retraction phases; air delivery passages (32, 34, 36) alignable between the feed port and the front air chamber; and wherein the air feed passage is a tube having a closed end mounted for relative axial movement within the piston; and the feed port is defined by at least one aperture in the tube wall adjacent the closed end; Characterized in a valve for the feed port in the form of a sleeve slidable around the air feed passage between back and front limit positions within the piston; whereby while the piston is advancing toward the bit during the actuation phase the sleeve is at the back limit position, closing said feed port, and when the piston impacts the bit said sleeve slides to the front limit position, opening said feed port and thereby delivering pneumatic pressure from the air feed passage through the air delivery passages (32, 34, 36) to the front air chamber to initiate the retraction phase.