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