snapping it past the end stop formation 37 and running it up the threads to a predetermined start position as shown in fig6 a and b the forward housing 10 1 is then snapped onto the rear housing 10 2 by a snap fit this action causes the front end of the drive shaft to enter into the drive gear and for the ratchet to engage but with the torsion spring unstrained next the drive shaft is rotated as shown in fig7 a to apply the required preload to the torsion spring the spring is held against rotation at its front end by the spring locating disk the forward end of the drive shaft rotates relative to the drive gear in ratchet fashion with the drive gear being held against rotational movement by the trigger having applied the preload the dose setting knob is aligned such that the zero mark lines up with the arrow on the housing and then snap fitted onto the end of the drive shaft the cartridge housing is then attached to the front end of the drive housing by suitable coupling action such as snap fit screw thread or bayonet as shown in fig8 a and b referring now to fig9 a to c in an alternative embodiment instead of using a separate spring locating disk 44 an internal annular wall 48 in the forward part of the rearward drive housing is provided with an opening 62 designed to capture and seat a spring hook 64 on the forward end of the torsion spring 24 when the torsion spring is introduced longitudinally into the housing and rotated in a yet further embodiment.