A drug delivery device for dispensing of a pre-defined amount of a liquid drug, the drug delivery device comprising: - a drive mechanism comprising a plunger, wherein the plunger comprises an end face comprising at least one circular or annular shaped receptacle, - a cartridge comprising a substantially cylindrical cartridge body and a piston, wherein the cartridge body provides a drug receiving volume at least partially filled with a medicament and sealed by the piston, wherein the piston is slidably disposed in the cartridge, the piston comprises: - a body comprising natural or synthetic rubber, - at least one annular sealing surface to radially abut against an inner side wall of the cartridge, - a distal surface to confine the drug receiving volume of the cartridge, - a thrust receiving surface adapted to receive the thrust exerting plunger (10, 32) for displacing the piston in a distal direction relative to the side wall, - wherein the thrust receiving surface comprises numerous centering elements protruding from the thrust receiving surface towards the plunger (10, 32) and wherein the centering elements are arranged and aligned on the circumference of an imaginary annular or circular structure, - wherein the centering elements comprise an outer shape and geometry that matches with the circular or annular shape of the receptacle of the plunger (10, 32), - wherein the shape of the receptacle corresponds with the alignment of the centering elements and wherein the receptacle is adapted to receive numerous centering elements, characterized in that the centering elements comprise an arcuate shape in a plane parallel to the thrust receiving surface and wherein in use of the cartridge in the drug delivery device the centering elements are entirely received in the receptacle such that a distal end face of the plunger (10, 32) almost entirely buts against the thrust receiving surface. - a drive mechanism comprising a plunger, wherein the plunger comprises an end face comprising at least one circular or annular shaped receptacle, - a cartridge comprising a substantially cylindrical cartridge body and a piston, wherein the cartridge body provides a drug receiving volume at least partially filled with a medicament and sealed by the piston, wherein the piston is slidably disposed in the cartridge, the piston comprises: - a body comprising natural or synthetic rubber, - at least one annular sealing surface to radially abut against an inner side wall of the cartridge, - a distal surface to confine the drug receiving volume of the cartridge, - a thrust receiving surface adapted to receive the thrust exerting plunger (10, 32) for displacing the piston in a distal direction relative to the side wall, - wherein the thrust receiving surface comprises numerous centering elements protruding from the thrust receiving surface towards the plunger (10, 32) and wherein the centering elements are arranged and aligned on the circumference of an imaginary annular or circular structure, - a body comprising natural or synthetic rubber, - at least one annular sealing surface to radially abut against an inner side wall of the cartridge, - a distal surface to confine the drug receiving volume of the cartridge, - a thrust receiving surface adapted to receive the thrust exerting plunger (10, 32) for displacing the piston in a distal direction relative to the side wall, - wherein the thrust receiving surface comprises numerous centering elements protruding from the thrust receiving surface towards the plunger (10, 32) and wherein the centering elements are arranged and aligned on the circumference of an imaginary annular or circular structure, - wherein the centering elements comprise an outer shape and geometry that matches with the circular or annular shape of the receptacle of the plunger (10, 32), - wherein the shape of the receptacle corresponds with the alignment of the centering elements and wherein the receptacle is adapted to receive numerous centering elements, characterized in that