A method of improving the lubrication system of a large slow running engine, wherein the engine comprises 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 lubrication system is configured for providing lubricant to the injectors in the injection phases; wherein the lubrication system comprises a lubricator pipe-connected to each injector by a lubricant feed conduit for providing pressurized lubricant to each injector though the lubricant feed conduit in the injection phases; wherein the lubricator comprises a housing in which a hydraulic-driven actuator piston is arranged reciprocating 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 valve-member pre-stressed against a valve-seat by the load of a spring; characterized in that the method comprises removing the spring and valve member from the valve chamber and modifying the valve-seat in the valve-chamber by working the valve seat with a working tool, the working tool displacing, removing, or adding material to the valve seat and improving dimensional precision of the valve seat by the working; subsequently operating the lubricator in the lubrication system of the engine with the modified valve-seat. wherein the engine comprises 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 lubrication system is configured for providing lubricant to the injectors in the injection phases; wherein the lubrication system comprises a lubricator pipe-connected to each injector by a lubricant feed conduit for providing pressurized lubricant to each injector though the lubricant feed conduit in the injection phases; wherein the lubricator comprises a housing in which a hydraulic-driven actuator piston is arranged reciprocating 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 valve-member pre-stressed against a valve-seat by the load of a spring; characterized in that the method comprises removing the spring and valve member from the valve chamber and modifying the valve-seat in the valve-chamber by working the valve seat with a working tool, the working tool displacing, removing, or adding material to the valve seat and improving dimensional precision of the valve seat by the working; subsequently operating the lubricator in the lubrication system of the engine with the modified valve-seat.