The present invention relates to an assembly forming a piston rod for advancing a cartridge bung (44) in a drug delivery device (10), the assembly comprising a first member (20) and a second member (22) wherein in a first state, the first (20) and the second member (22) are moveable with respect to each other to adjust the length of the piston rod and wherein in a second state the first member (20) is locked in position with respect to the second member (22) in order to define a fixed length of the piston rod. Furthermore, it relates to a drug delivery device (10) comprising the assembly and to a method for assembling such a drug delivery device (10).
