a restoring spring 28 that is supported on the housing block 23 the piston segment 26 is either firmly connected to the cup shaped cam piston 27 or as shown here is held form fittingly on the cam piston 27 via the same restoring spring 28 the axial housing bore 24 the valve piston 25 and the piston segment 26 define a stroke transmission chamber 29 that is filled with a pressure fluid in this case oil its effective axial length between the cam piston 27 and the valve piston 25 can be varied by relative motion of the pistons to one another the stroke transmission chamber 29 communicates via a line 30 with a magnetic control valve 31 on the one hand and with a supply container 32 on the other a one way check valve 33 and a feed pump 34 are incorporated between the line 30 and the supply container 32 via the line 30 the volume of oil present in the stroke transmission chamber 29 can be displaced into a spring reservoir of the magnetic control valve 31 and back again leakage losses in the oil volume are made up from the supply container 32 via the feed pump 34 and the check valve 33 the magnetic valve 31 controls the oil volume and hence the axial length of the stroke transmission chamber.