operable linear motor controlled by an encoder which responds to displacement of the motor a modified embodiment 100 is illustrated in fig1 and 11 in embodiment 100 the piston chamber is defined by the internal bore 118 of a hollow tube 120 the hollow tube 120 is provided with a plurality of communication bores 121 extending generally radially through the wall of the tube 120 the bores 121 are spaced along the length of the tube and are arranged such that each bore 121 is aligned with a corresponding cylinder bore 16 so as to provide fluid communication between the corresponding bore 16 and the piston chamber the tube 120 is conveniently made from a suitable plastics material such as a polyamide accordingly the tube 120 is simple to manufacture by for example extrusion techniques to define the piston chamber drilling of the tube wall is conveniently performed in order to define the communication bores 121 the support body 14 in embodiment 100 includes an elongate recess 130 which defines a seat for the tubes 120 the recess 130 is preferably part circular in cross section having a diameter corresponding to the outer diameter of tube 120 terminal ends of the piston cylinders 16 open into the recess 130 accordingly when the tube 120 is seated in the recess 130 its outer face is in face to face contact with.