The stator comprises a rigid tubular casing and an elastic lining which is tightly enclosed by the casing and symmetrical with respect to the longitudinal center axis of the stator. The inner surface of the lining forms a multiple thread and, in cross section, consists of a number of sections near the axis equal to the number of courses of the thread and an equally great number of concave arcs. The lining has a minimum thickness each at the concave arcs and a maximum thickness each near at least two of the transitions between the sections near the axis and the concave arcs.