The present invention relates to a piston washer assembly (100, 100′) for use in a drug delivery device for transferring a distally directed axial force from a piston rod (220) towards a piston (214) of a held cartridge (213). The piston washer assembly (100, 100′) comprises a) a proximal part (110, 110′) adapted for engagement with the distal portion of a piston rod (220), the proximal part (110, 110′) defining a first set of cooperating geometries (115, 115′), and b) a distal part (120, 120′) configured to engage and abut a piston (214) of a held cartridge (213) and to engage the proximal part (110, 110′), wherein the distal part (120, 120′) defines a second set of cooperating geometries (125, 125′) engaging the first set of cooperating geometries (115, 115′). In an initial state of the assembly, the proximal part (110, 110′) and the distal part (120, 120′) are positioned relatively to each other to define an initial axial dimension (H1) for the assembly. The first set of cooperating geometries (115, 115′) and the second set of cooperating geometries (125, 125′) are configured to enable relative movement for increasing the axial dimension for obtaining a target axial dimension (H2) but preventing a decrease in axial dimension. The invention further relates to a drug delivery device, a method of assembling a drug delivery device incorporating such piston washer assembly and a method of preparing a piston washer assembly.
