Rod-locking unit (1) for a rotary printing machine comprises a rod-receiving element (9) forming a hollow body and surrounding a bearing (11) receiving a printing roller rod (13) with a rod bearing surface (12). The locking unit is displaced between a receiving position in which the printing roller rod engages with the bearing and a release position in which the printing roller rod disengages from the bearing. The locking unit also comprises a cylinder (2) containing a pressure chamber (3) and a piston (4) for displacing the receiving element between the receiving and release positions. The piston defines the pressure chamber at a boundary and at a connecting point with the receiving element to transfer the displacement force required. The distance between the boundary and the connecting point is shorter than the maximum stroke of the piston in the cylinder. The cylinder has an opening (14) that is open in the release position so that the printing roller rod and the locking unit are separated by displacement relative to each other.