The invention relates to an auto-injector (1) for administering a dose of a liquid medicament (M), comprising: - an elongate housing (2), a syringe (7) with a hollow needle (9) wherein the syringe (7) is slidably arranged with respect to the housing (2), - spring means (14) for pushing the needle (9) from a covered position inside the housing (2) into an advanced position past the proximal end (P) and for supplying the dose of medicament (M), - activating means (4) for locking and releasing the spring means (14). The spring means (14) is a torsion spring (14). A lead nut (15) is arranged for translatively moving a piston rod (11) by means of a lead screw thread (16). Prior to manual operation a trigger button (4), the piston rod (11) and the lead nut (15) are coupled for joint translation in proximal direction (P). In the initial position the lead nut (15) is engaged to the housing (2) by at least one spline feature (41). The lead nut (15) is arranged to disengage from the housing (2) on translation in proximal direction (P) from the initial position.
