The present invention relates to a drive mechanism for a drug delivery device for dispensing of a dose of a medicinal product, comprising: a holder for a product-containing cartridge, the cartridge having a piston slidably arranged therein in an axial direction, a piston rod to be operably engaged with the cartridge's piston for dispensing of a dose of the medicinal product, an adjusting assembly adapted to eliminate axial clearance between the piston and the piston rod, wherein the adjusting assembly comprises at least one radially extending spike element for mutually locking in position the piston and the piston rod irrespective of the relative distance between piston and piston rod.