PURPOSE: To facilitate machining by providing a second groove on the peripheral surface of a piston between a helical groove means on the piston peripheral surface and a piston part adjacent to the cylinder closing end, providing a cutaway part communicated with the second groove on the cylinder wall, and making the cutaway part coaxial with a gutter port.

CONSTITUTION: A helical groove means 18 communicated with a closing end 19 of a cylinder 14 is provided on the peripheral surface of a piston 16. Additional means 22, 28 are provided for communicating a gutter port 17 with the closing end 19 of the cylinder 14 when the piston 16 is in a position C-D between a position A-B in which the gutter port 17 is directly communicated with the closing end 19 of the cylinder 14 and a position E in which the gutter port 17 is communicated with the helical groove means 18. This additional means 28 is a second groove provided on the peripheral surface of the piston 16 between the helical groove means 18 and the piston 16 adjacent to the closing end 19 of the cylinder 14, and the additional means 22 is coaxial with the axis of the gutter port 17 by a cutaway part provided on the wall of the cylinder 14 and to be communicated with the second groove 28. Therefore, the gutter port 17 and the cutaway part 22 can be formed by the same rotary drill.