into which a crosspin 27 is diametrically pressed directly below the ball bearing 25 the ends of the crosspin protruding from the shaft 26 are received by a groove 28 of the tubular extension 24 the groove 28 extends azimuthally over an angle that is larger than the adjusting range of the rotary adjuster 13 see also fig5 the rotary adjuster 13 inserted into the piston housing recess 12 comprises a stator 30 and a permanent magnetic rotor 31 the stator has a stator lamination packet 32 and a winding 33 with a winding lead 34 the individual stator parts are shown again in fig2 which is a cross section through the stator lamination packet 32 the packet has two oppositely disposed teeth 35 and 36 two grooves 37 and 38 and a magnetic short circuit 39 a coil 40 which is divided into two halves 41 and 42 is inserted into the grooves 37 and 38 the two coil halves 41 and 42 are connected in series and each is wound about a respective tooth 35 and 36 the coil winding 33 is laid into the grooves through the openings 43 and 44 in the grooves 37 and 38 fig3 a and 3b show the rotor 31 of the rotary adjuster 13 it is embodied as a hollow cylinder 45 which is.