An intermediate section is provided with a twistable zone which has several axially running grooves distributed over the periphery. The grooves (15) in the area of the twistable zone (7) have a groove profile constant in an axial direction as to dim. and shape. The twistable zone extends over a part length or the complete length of the intermediate section (6). Between adjacent grooves an axially running rib (16) is formed. Each rib in the area of the twistable zone has a constant rib profile in an axial direction as to dim. and shape. The cross-sectional profile of the torsion area (twistable zone) is constant over its length.
