Abstract: A reciprocating percussion instrument has a housing forming an axially centered chamber having a front end and formed thereat with a pair of adjacent ports, a tool projecting from the housing and exposed axially in the chamber, and a double-acting piston forming front and back compartments and axially reciprocatable between a rear end position and a front end position at least generally in axial engagement with the piston. The piston has a groove forming a fluid-connection path in the chamber between the ports only in the front end position of the piston. A conduit connects one of the ports to one of the sides of the pressure source.