also fig3 4 and 6 of a piston 3 according to the invention has an annular shape and in particular a standpipe like inflow 4 and in particular a standpipe like outflow 5 the inflow 4 and the outflow 5 in this case are formed unitarily with the lower covering 1 in particular bent out or formed from the covering 1 or at least partially formed by a bent up metal sheet section 10 according to fig1 the entire lower covering 1 is formed unitarily whereas the covering 1 according to fig2 consists of two semi annular sections 6 7 on the face ends of which a half of the inflow 4 and of the outflow 5 is bent out so that upon a joining of the two sections 6 7 two adjacent sections 6 7 each form an inflow 4 and an outflow 5 at the face end between them see also fig5 in this regard here the covering 1 is preferentially embodied as punched shaped sheet metal part wherein the inflow 4 and or the outflow 5 is are bent out of the covering 1 the bending out or the forming of the metal sheet section 10 forming the inflow 4 and or the outflow 5 in this case.