A method of assembling a drug delivery device, the drug delivery device comprising: - a first housing component adapted to receive and to house a cartridge, the cartridge comprising a medicament and a piston slidably arranged therein in an axial direction, - a second housing component adapted to house a drive mechanism, the drive mechanism comprising a piston rod assembly which is variable in length and which is adapted to modify an axial gap between the piston rod assembly and the piston of the cartridge to a predefined gap size, wherein the piston rod assembly comprises a piston rod and an adjusting member, the piston rod being operably engagable with the piston and the adjusting member being interconnected with a distal end section of the piston rod the method comprising the steps of: - positioning the cartridge being filled with the medicament in the first housing component to form a cartridge sub-assembly, - positioning the drive mechanism comprising the piston rod assembly in the second housing component to form a housing sub-assembly, characterized by - determining the axial position of the piston with respect to the first housing component, - determining the axial position of the piston rod assembly with respect to the second housing component, - modifying the axial dimensions of the piston rod assembly by moving the adjusting member relative to the piston rod such that the axial distance between the piston and the piston rod assembly equals the predefined gap size when the drug delivery device is finally assembled, - mutually interconnecting first and second housing components (152, 154) after having modified the relative axial position of adjusting member and piston rod. - a first housing component adapted to receive and to house a cartridge, the cartridge comprising a medicament and a piston slidably arranged therein in an axial direction, - a second housing component adapted to house a drive mechanism, the drive mechanism comprising a piston rod assembly which is variable in length and which is adapted to modify an axial gap between the piston rod assembly and the piston of the cartridge to a predefined gap size, wherein the piston rod assembly comprises a piston rod and an adjusting member, the piston rod being operably engagable with the piston and the adjusting member being interconnected with a distal end section of the piston rod the method comprising the steps of: - positioning the cartridge being filled with the medicament in the first housing component to form a cartridge sub-assembly, - positioning the drive mechanism comprising the piston rod assembly in the second housing component to form a housing sub-assembly, characterized by - determining the axial position of the piston with respect to the first housing component, - determining the axial position of the piston rod assembly with respect to the second housing component, - modifying the axial dimensions of the piston rod assembly by moving the adjusting member relative to the piston rod such that the axial distance between the piston and the piston rod assembly equals the predefined gap size when the drug delivery device is finally assembled, - mutually interconnecting first and second housing components (152, 154) after having modified the relative axial position of adjusting member and piston rod.