A mandrel-locking unit for a rotary printing machine has a mandrel-mounting element that surrounds print roller mandrels in an inner bearing for mounting a mandrel-supporting surface and into which the mandrel-mounting element can be slid such that the mandrel-supporting surface is released by the sliding movement. The unit has a pressurizing medium cylinder that provides the force required for the sliding movement and a piston, which delimits a pressure chamber of the pressurizing medium cylinder at a boundary surface and which is in contact with the mandrel-mounting element at a connecting point and conveys to the mandrel mounting element the pressure required for the sliding movement. A feature of the unit is that the distance between the boundary surface and the connecting point is smaller than the maximum stroke of the piston in the pressurizing medium cylinder.