A lubricator for a large slow running engine, the engine comprising a cylinder with a reciprocal piston inside and with a plurality of lubricant injectors distributed along a perimeter of the cylinder for injection of lubricant into the cylinder at various positions on the perimeter during injection phases; wherein the lubricator is configured for providing pressurized lubricant to the injectors in the injection phases; wherein the lubricator comprises a housing in which a hydraulic-driven actuator piston is arranged reciprocal along a stroke length, wherein the housing further contains a plurality of injection plungers and a corresponding plurality of dosing channels as well as non-return valves , one for each dosing channel , each injection plunger being slidingly arranged in one of the dosing channels , wherein the injection plungers are coupled to the actuator piston in order to be moved in common by the actuator piston over the stroke length and as a consequence pressurize lubricant in the dosing channels over a dosing channel distance for expelling the pressurized lubricant from each dosing channels through the corresponding non-return valve for injection of the lubricant into the engine cylinder ; wherein the dosing channel distance defines the volume of lubricant expelled from the dosing channel through the non-return valve during the injection phase; wherein each non-return valve is provided in a corresponding valve-chamber in the housing and comprises a spherical valve-member pre-stressed against a valve-seat by the load of a spring ; wherein the valve seat has a diameter of 3 to 10 mm; wherein the thickness of the ring-formed contact surface between the valve member and the valve seat is in the range of 0.01 mm to 0.5 mm, characterized in that the valve-seat has a circular symmetric shape wherein the circular symmetry varies by less than 0.2 mm when measured across the valve seat; wherein the surface roughness of the valve seat is less than 12.5 μm. wherein the lubricator comprises a housing in which a hydraulic-driven actuator piston is arranged reciprocal along a stroke length, wherein the housing further contains a plurality of injection plungers and a corresponding plurality of dosing channels as well as non-return valves , one for each dosing channel , each injection plunger being slidingly arranged in one of the dosing channels , wherein the injection plungers are coupled to the actuator piston in order to be moved in common by the actuator piston over the stroke length and as a consequence pressurize lubricant in the dosing channels over a dosing channel distance for expelling the pressurized lubricant from each dosing channels through the corresponding non-return valve for injection of the lubricant into the engine cylinder ; wherein the dosing channel distance defines the volume of lubricant expelled from the dosing channel through the non-return valve during the injection phase; wherein each non-return valve is provided in a corresponding valve-chamber in the housing and comprises a spherical valve-member pre-stressed against a valve-seat by the load of a spring ; wherein the valve seat has a diameter of 3 to 10 mm; wherein the thickness of the ring-formed contact surface between the valve member and the valve seat is in the range of 0.01 mm to 0.5 mm, characterized in that the valve-seat has a circular symmetric shape wherein the circular symmetry varies by less than 0.2 mm when measured across the valve seat; wherein the surface roughness of the valve seat is less than 12.5 μm. <EOS>