Method for assembling a drug delivery device or components thereof comprising the steps: a) providing a housing (10, 20, 150), a cartridge with a bung, a plunger, a retaining member coupled to the plunger, a spring biasing the plunger towards the bung, a release member coupled to the spring such that release of the spring is controlled by the release member, and a dose setting and dispensing mechanism comprising at least a dose setting member, the release member and/or a drum, the plunger and the spring, which is a strained pressure spring, wherein the cartridge with the bung, the dose setting and dispensing mechanism, the plunger and the spring are mounted within the housing (10, 20, 150) such that the spring biases the plunger in a dispensing direction and that the drum and/or the release member is attached to the plunger, and wherein the retaining member is attached to and wound on the drum which is in gear engagement with the release member or the retaining member is directly attached to and wound on the release member, wherein winding the retaining member onto the drum and/or the release member generates a torque in these component parts as the spring acts on the plunger, b) providing a tool comprising an instrument for measuring the torque of the spring reacted by the release member, characterized in that the dose setting and dispensing mechanism further comprises a trigger and in that the method further comprises the steps: c) releasing the spring while monitoring the torque by the measuring instrument, d) stopping the release member upon detection of a predetermined change in the torque measured by the measuring instrument, wherein the predetermined change in the torque is a rapid decrease due to the plunger contacting the bung in the cartridge, wherein e) the tool with torque measurement capability is attached to the drum or the release member of the dose setting and dispensing mechanism in a state where the drum or the release member is accessible from outside the housing (10, 20, 150), f) thereafter a dose is set by rotating the dose setting member in a first direction, thereafter the trigger is activated and the torque applied to the drum or the release member is monitored, g) the trigger is released upon detection of a predetermined change in the torque measured by the tool, h) the tool is released from the drum or the release member. a) providing a housing (10, 20, 150), a cartridge with a bung, a plunger, a retaining member coupled to the plunger, a spring biasing the plunger towards the bung, a release member coupled to the spring such that release of the spring is controlled by the release member, and a dose setting and dispensing mechanism comprising at least a dose setting member, the release member and/or a drum, the plunger and the spring, which is a strained pressure spring, wherein the cartridge with the bung, the dose setting and dispensing mechanism, the plunger and the spring are mounted within the housing (10, 20, 150) such that the spring biases the plunger in a dispensing direction and that the drum and/or the release member is attached to the plunger, and wherein the retaining member is attached to and wound on the drum which is in gear engagement with the release member or the retaining member is directly attached to and wound on the release member, wherein winding the retaining member onto the drum and/or the release member generates a torque in these component parts as the spring acts on the plunger, b) providing a tool comprising an instrument for measuring the torque of the spring reacted by the release member, characterized in that the dose setting and dispensing mechanism further comprises a trigger and in that the method further comprises the steps: c) releasing the spring while monitoring the torque by the measuring instrument, d) stopping the release member upon detection of a predetermined change in the torque measured by the measuring instrument, wherein the predetermined change in the torque is a rapid decrease due to the plunger contacting the bung in the cartridge, wherein e) the tool with torque measurement capability is attached to the drum or the release member of the dose setting and dispensing mechanism in a state where the drum or the release member is accessible from outside the housing (10, 20, 150), f) thereafter a dose is set by rotating the dose setting member in a first direction, thereafter the trigger is activated and the torque applied to the drum or the release member is monitored, g) the trigger is released upon detection of a predetermined change in the torque measured by the tool, h) the tool is released from the drum or the release member.