The present invention relates a method of assembling a drug delivery device the method comprising the steps of:- positioning the cartridge (153) being filled with the medicament, in the first housing component (152) to form a cartridge sub-assembly,- positioning the drive mechanism comprising the piston rod assembly in the second housing component (154) to form a housing sub-assembly,- determine the axial position of the piston (155) with respect to the first housing component (152),- determining the axial position of the piston rod assembly with respect to the second housing component (154),- modifying the axial dimensions of the piston rod assembly by moving the adjusting member (202) relative to the piston rod (200) such that the axial distance between the piston (155) 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).
