600595 A piston for a cartridge 24 for a drug delivery device comprises a body comprising natural or synthetic rubber. The piston comprises at least one annular sealing surface 35 to radially abut against an inner side wall 24 of a cartridge 33, a distal surface 42 to confine a drug receiving volume of the cartridge 33, a substantially planar thrust receiving surface 40 adapted to receive a thrust exerting plunger 32 of a drug delivery device for displacing the piston in a proximal direction relative to the side wall 24. The thrust receiving surface 40 comprises numerous centring elements 34 protruding from the thrust receiving surface 40 towards the plunger 32. The centring elements 34 comprise an outer shape and geometry that matches with a corresponding receptacle 38 of the plunger 32. The centring elements 34 comprise an arcuate shape in a plane parallel to the thrust receiving surface 40 and are arranged and aligned on the circumference of an imaginary annular or circular structure. At least one centring element comprises a knob-like shape.
