of the cylinder 1 when the screw 10 is loosened the guide bush 7 may be rotated relative to the cylinder 1 to a desired position of adjustment thereafter the screw is tightened causing the end of the cylinder to be clamped between the alignment member 8 and the bush 7 thereby fixing the bush relative to the cylinder the end portion 7 of the guide bush 3 is of non circular contour being provided with a collar or extension 12 two pockets 13 see fig2 are provided in the extension 12 each pocket accommodating a sensor 14 such as a magnetic or proximity switch the pockets 13 are offset from each other by 90 u00b0 in the direction of the periphery of the piston rod 2 so that the sensors 14 are aligned with the two rows of permanent magnets 5 the alignment member 8 holds the sensors 14 in precise alignment with the two rows of permanent magnets 5 the ends 15 of the pockets 13 that are presented to the piston rod 2 are open so that the sensors 14 are close to the piston rod the ends of the pockets 13 adjacent to the outer periphery of the end.