A method for assembling a drug delivery device comprising a body (2) retaining a piston rod (3) and a cartridge (5) retaining a bung (6) and holding a drug, said body (2) having a distal end (11) and a proximal end (12), comprises the following steps: A) Providing the cartridge (5) and the body (2) retaining the piston rod (3), wherein the piston rod (3) is plastically deformable, at least a part of said plastically deformable piston rod being manufactured from a plastically deformable material, B) Applying a force on the piston rod (3), at least a part of the piston rod (3) being in a state that allows plastic deformation, so that the piston rod (3) is plastically deformed and an element of the piston rod (3) abuts the bung (6), C) Applying conditions in order to bring the plastically deformed piston rod (3) of step B) into an altered state wherein no further plastic deformation takes place, D) Securing the cartridge (5) to the body (2).
