complementary manner the outside of the piston 6 bas a first portion 26 carrying a set of helical teeth 27 meshed with the teeth 25 the piston also has a second portion 28 the free end of which has a flange 29 extending radially outwardly and engaged sealingly with the portion 20 of the skirt 16 with the aid of a piston ring 30 the piston 6 together with the hub 2 and the half bodies 10 11 defines an annular chamber indicated 33 braking means are provided between the hub 2 and the casing 9 these means comprise a ring 34 having a conical outer surface and a cylindrical inner surface and restrained axially on the hub 2 the half body 11 in turn has a conical inner surface in contact with the conical outer surface of the ring 34 the conical coupling between the two conical surfaces of the ring 34 and of the half body 11 is indicated 35 the hub 2 has an axial cavity 41 closed at one end by the cover 17 and extended at its opposite end by a feed duct 51 for pressurized oil or other servo meads for the operative control of the phase variator with a branch directed through the structure b of the.