the distal portion 11 b which extends through the opening 18 in the transverse wall 17 of the casing has a prismatic shape or has one or more flat surfaces 11 c whose function is described hereinafter and a terminal portion 11 d for connection to the distributor r as will be seen the rod and the nut co operate by performing a telescopic movement with respect to the fixed casing as an alternative to the example illustrated the thread 20 which is fixedly joined to the casing 12 could be formed in a single piece with the casing instead of being formed by a separate element the bush 19 the nut element 22 has a base portion in the form of a radial flange 25 in which are formed axially oriented through openings 26 in which the pins 16 carried by the rotary member 15 engage slidingly in the region of the outlet opening 18 for the rod 11 the casing has means for preventing the rotation of the rod about the longitudinal axis of the actuator those anti rotation means may be constituted by the opening 18 itself which may have a non circular shape corresponding to the cross section of the distal portion 11 b of the.