A piston designed for use in metering machines employed in the food, chemical and pharmaceutical industries. The piston preferably has a cylindrical body with an annular bead in an intermediate position, a cap made of material designed to form a seal against the cylinder wall, which cap is fitted onto the body, and an annular support with an expansion spring, housed in a groove in the inner wall of the cap, the cap having an annular bead on the free edge; a clamping ring with a groove designed to fit onto the annular bead of the cap and onto the annular bead on the piston body is provided to keep the elements joined together, and the ring also acts as a guide for the piston against the cylinder wall.