A hammer drill comprising; a housing; a tool holder mounted on the housing which is capable of holding a cutting tool; a motor mounted within the housing; and a hammer mechanism comprising: a piston reciprocatingly driven along a longitudinal axis by the motor when the motor is actuated; a ram reciprocatingly driven on the longitudinal axis by the reciprocating piston via an air spring; a beat piece supported in an axially slideable manner on the longitudinal axis within a beat piece support structure (150, 210) which, during the normal operation of the hammer mechanism, is repetitively struck by the ram and which transfers the impacts to a cutting tool when held by the tool holder; wherein the beat piece comprises a first impact surface and the beat piece support structure (150, 210) comprises a second impact surface, the first and second impact surfaces (248, 250) coming into contact with each other when the beat piece axially slides to its furthest position away from the ram; characterised in that the shape of the first and second impact surfaces (248, 250) relative to each other is arranged so that there is a non uniform amount of contact between the first and second impact surfaces (248, 250) around the longitudinal axis; wherein either: the first impact surface is circular in cross section in any plane which extends perpendicularly from a first axis which extends in a direction parallel to the longitudinal axis, the centres of the circular cross sections being located on the first axis; and the second impact surface is circular in cross section in any plane which extends perpendicularly from the longitudinal axis, the centres of the circular cross sections being located on the longitudinal axis, the first axis being off set relative to the longitudinal axis; or, the first impact surface is circular in cross section in any plane which extends perpendicularly from the longitudinal axis, the centres of the circular cross sections being located on the longitudinal axis; and the second impact surface is circular in cross section in any plane which extends perpendicularly from a second axis which is parallel to the longitudinal axis, the centres of the circular cross sections being located on the second axis, the second axis being located in close proximity to the longitudinal axis. a piston reciprocatingly driven along a longitudinal axis by the motor when the motor is actuated; a ram reciprocatingly driven on the longitudinal axis by the reciprocating piston via an air spring; a beat piece supported in an axially slideable manner on the longitudinal axis within a beat piece support structure (150, 210) which, during the normal operation of the hammer mechanism, is repetitively struck by the ram and which transfers the impacts to a cutting tool when held by the tool holder; wherein the beat piece comprises a first impact surface and the beat piece support structure (150, 210) comprises a second impact surface, the first and second impact surfaces (248, 250) coming into contact with each other when the beat piece axially slides to its furthest position away from the ram; characterised in that the shape of the first and second impact surfaces (248, 250) relative to each other is arranged so that there is a non uniform amount of contact between the first and second impact surfaces (248, 250) around the longitudinal axis; wherein either: the first impact surface is circular in cross section in any plane which extends perpendicularly from a first axis which extends in a direction parallel to the longitudinal axis, the centres of the circular cross sections being located on the first axis; and the second impact surface is circular in cross section in any plane which extends perpendicularly from the longitudinal axis, the centres of the circular cross sections being located on the longitudinal axis, the first axis being off set relative to the longitudinal axis; or, the first impact surface is circular in cross section in any plane which extends perpendicularly from the longitudinal axis, the centres of the circular cross sections being located on the longitudinal axis; and the second impact surface is circular in cross section in any plane which extends perpendicularly from a second axis which is parallel to the longitudinal axis, the centres of the circular cross sections being located on the second axis, the second axis being located in close proximity to the longitudinal axis. the first impact surface is circular in cross section in any plane which extends perpendicularly from a first axis which extends in a direction parallel to the longitudinal axis, the centres of the circular cross sections being located on the first axis; and the second impact surface is circular in cross section in any plane which extends perpendicularly from the longitudinal axis, the centres of the circular cross sections being located on the longitudinal axis, the first axis being off set relative to the longitudinal axis; or, the first impact surface is circular in cross section in any plane which extends perpendicularly from the longitudinal axis, the centres of the circular cross sections being located on the longitudinal axis; and the second impact surface is circular in cross section in any plane which extends perpendicularly from a second axis which is parallel to the longitudinal axis, the centres of the circular cross sections being located on the second axis, the second axis being located in close proximity to the longitudinal axis.