A dosing unit for an infusion pump device is disclosed having a piston pump with a pump cylinder and a plunger arranged within said cylinder, coaxially arranged along a longitudinal axis. The plunger has a shaft with a thread and the cylinder has a threaded sleeve part with a thread. The two threads engaging with each other in such a way that a rotational movement of the plunger around the longitudinal axis results in an additional linear displacement of the plunger along said longitudinal axis. A separate bias force element biases the two threads in regard to each other along the longitudinal axis, such that the threaded engagement of inner thread and outer thread is free of play independent of a direction of a rotational movement and linear displacement of the plunger in regard to the cylinder.
