The invention concerns a piston wherein the mobile screw (2) is engaged in the bolt (8) integral with the mobile assembly (1). It is provided with a cylindrical axial housing (10) wherein is engaged the rod (7) integral at its front end with the expander assembly (4). At the end of its front travel, the rod is retained by its flange (19) pressed on the stop (11) such that the expanding ring (5) is compressed by the thrust of the piston body (3) and causes the flexible envelope (6) to expand.