A hammer drill comprising: a body; a cylinder mounted within the body; a piston slideably mounted within the cylinder; a motor mounted within the body having an output spindle; a reciprocating drive mechanism mounted within the body which, when the motor is activated, is able to convert the rotary movement of the spindle of the motor into a reciprocating movement of the piston within the cylinder; a ram slideably mounted within the cylinder which is reciprocatingly driven by the reciprocating movement of the piston via an air spring; at least two air entry points (200, 202) formed through the wall of the cylinder in different axial positions along the length of the cylinder, each of which are capable of replenishing the air spring; a tool holder mounted on the body, the tool holder being capable of holding a cutting tool which, during normal use, when the hammer drill is running continuously whilst working on a work piece, is repetitively struck by the reciprocating ram; wherein there is further provided a selector mechanism which is configured to selectively open and close the air entry points (200, 202); wherein the selector mechanism comprises a sleeve mounted in an axially slideable manner on the cylinder; characterised in that the sleeve comprises at least two apertures (206, 208) which are capable of being selectively aligned with one or more of the air entry points (200, 202) as the sleeve is axially slid along the cylinder, the sleeve closing the remaining air entry points (200, 202); whereby the selector mechanism opens only one air entry point (200, 202) at any one time during normal use, when the hammer drill is running continuously whilst working on a work piece, the selector mechanism closing all other air entry points (200, 202), each of the air entry points (200, 202) being capable of replenishing the air spring during normal use when opened by the selector mechanism; wherein the dimensions of the at least two air entry points (200, 202) are different relative to each other. a body; a cylinder mounted within the body; a piston slideably mounted within the cylinder; a motor mounted within the body having an output spindle; a reciprocating drive mechanism mounted within the body which, when the motor is activated, is able to convert the rotary movement of the spindle of the motor into a reciprocating movement of the piston within the cylinder; a ram slideably mounted within the cylinder which is reciprocatingly driven by the reciprocating movement of the piston via an air spring; at least two air entry points (200, 202) formed through the wall of the cylinder in different axial positions along the length of the cylinder, each of which are capable of replenishing the air spring; a tool holder mounted on the body, the tool holder being capable of holding a cutting tool which, during normal use, when the hammer drill is running continuously whilst working on a work piece, is repetitively struck by the reciprocating ram; wherein there is further provided a selector mechanism which is configured to selectively open and close the air entry points (200, 202); wherein the selector mechanism comprises a sleeve mounted in an axially slideable manner on the cylinder; characterised in that the sleeve comprises at least two apertures (206, 208) which are capable of being selectively aligned with one or more of the air entry points (200, 202) as the sleeve is axially slid along the cylinder, the sleeve closing the remaining air entry points (200, 202); whereby the selector mechanism opens only one air entry point (200, 202) at any one time during normal use, when the hammer drill is running continuously whilst working on a work piece, the selector mechanism closing all other air entry points (200, 202), each of the air entry points (200, 202) being capable of replenishing the air spring during normal use when opened by the selector mechanism; wherein the dimensions of the at least two air entry points (200, 202) are different relative to each other.