Mechanism for a drug delivery device

An assembly forming a piston rod for advancing a cartridge bung in a drug delivery device, the piston rod comprising:

• an adjustment member comprising an aperture;
• a first member operatively engaged with the adjustment member such that the first member extends through the aperture and can move axially relative to the adjustment member when the piston rod is in a first state;
• and
• a second member, where the first member and second member are axially and moveably connected to each other along a common longitudinal axis and through the adjustment member to define a first length when the piston rod is in the first state,
• wherein when the piston rod is in the first state the second member does not move axially and the first member is axially moveable with respect to the second member as a result of movement of the adjustment member so that the first length of the piston rod can be increased to a fixed length, and
• wherein when the piston rod is in a second state,
• the first member is locked in position with respect to the second member in order to define the fixed length of the piston rod, where the locked position forms a rigid connection through threaded engagement between the first member and the second member such that the fixed length of the piston rod does not change when a dose of a fluid medicinal product is set and then dispensed;
• and
• the adjustment member is locked with the second member in order to prevent rotational and axial movement with respect to the second member.