PISTON WASHER FOR A DRUG DELIVERY DEVICE, METHOD OF PREPARING SUCH PISTON WASHER AND METHOD OF ASSEMBLY OF A DRUG DELIVERY DEVICE

Described is a method of preparing a piston washer (100) for use in a drug delivery device and for transferring a distally directed axial force from a piston rod towards a piston of a held cartridge. The method comprises the steps of: a) providing a piston washer (100) comprising a deformable portion made from a plastically deformable material, the piston washer defining a proximal surface geometry (114) configured for engagement with the distal portion of a piston rod and a distal surface geometry (126) configured for engagement with the proximal surface of a piston of a cartridge wherein the proximal surface geometry (114) and the distal surface geometry (126) are arranged axially relative to each other with an initial axial spacing, b) arranging the piston washer in a tool arrangement (300), and c) using the tool arrangement (300) to apply plastical deformation of the deformable portion of the piston washer (100) thereby modifying the axial spacing between the proximal surface geometry and the distal surface geometry until a target axial spacing is obtained.