is a downwardly projecting boss or lug 3b fig1 and 3 extending over a definite length in the circumferential direction of the piston the lug 3b also constituting a part of the connecting means 3 on the underside of lug 3b there is an inclined surface 3c gradually rising or falling in the circumferential direction the inclination of the inclined surface 3c is desirably slight and its face is desirably wide it is necessary that at least one lug 3b and one inclined surface 3c be provided circumferentially of the piston but a plurality of them can be provided the body 2 is a cylindrical piece with an integral top end having an axially extending annular groove 2a to receive the projection 1a of the head 1 an annular radial groove 2b is formed in the inner side wall of groove 2a and opens into groove 2a groove 2b constitutes an engaged part of connecting means 3 in groove 2b there is a split ring 4 of a vertical height to be a very close fit in the groove 2b ring 4 fig2 constitutes an engaging part of a cooperating connecting means having a portion extending into groove 2a it is for the purpose of enabling the ring 4 to fit into groove 2a that the ring.