The piston rod arrangement is in an assembled state before use and comprises a piston rod, a bung, which is intended to be driven by the piston rod, a body member, and a drive member. The piston rod is threadedly engaged with the body member, and the drive member is threadedly engaged with the piston rod and movable relative to the body member. A mechanism is provided defining unit steps of movement of the drive member and corresponding unit steps of movement of the piston rod. The piston rod is arranged in contact with the bung or at a distance from the bung that is less than a distance of one corresponding unit step of movement. When the piston rod, the bung and the drive member are assembled in the body member, the piston rod is advanced with respect to the body member towards the bung, until the piston rod is stopped by the bung. The drive member is rotated until the piston rod begins to advance again and the drive member is then stopped, and the piston rod is fixed at or near the position obtained.