Piston, in particular for sampling valve

A piston, for a draw-off valve, comprising a piston body covered externally with an extensible casing, a displacement screw provided with rotational driving means and cooperating with a nut immovably attached to the piston body for displacement thereof in response to the rotations of the screw, and a dilating device linked to the piston and causing, at the limit of travel, a dilation of the casing, wherein the dilating device comprises a dilating disk and a rod having one end immovably attached to the disk, and wherein the screw has on the one hand a longitudinal housing in which the rod is fitted and on the other hand at its front end a limit stop which cooperates at the limit of travel with a collar of the rod situated at its back end, so as to cause a squeezing of the dilating disk against the end of the piston body and a radial expansion of said dilating device.