Abstract: A compressor has flat cylinder heads in which circular valves are disposed. The valves are of the type having a slotted valve seat and a slotted valve guard, with elongated valve elements disposed between the seat and the guard to open and close the slots of the valve seat. The flat circular valves are perpendicular to the piston axis and extend substantially farther from the piston axis than does the piston and so directly overlie only a portion of the piston. Therefore, the slots are provided only on the radially inner portion of the valve, the radially outer portion being imperforate. This enables the valve to be used with pistons of different diameters and under various conditions of operating pressure. Also, the cylinder liner is provided with a port at the intersection of its cylindrical inner side wall and its end wall that confronts the valve, whereby part of the clearance volume extends radially outwardly under those slots which would otherwise be masked by the liner.