The screw plug K 43591 is an important threaded closure component associated with part K 43553 within the automatic starting-air stop valve assembly of the Adriadiesel/Jugoturbina/Zgoda/Sulzer ZV40/48 engine. Its primary function is to securely close a designated threaded opening in the corresponding component, helping maintain the mechanical integrity and, where applicable, the pressure tightness of the valve assembly during starting-air system operation. Depending on the specific design and function of part K 43553, the screw plug may also provide access for assembly, inspection, adjustment, or maintenance while ensuring reliable closure during normal service. Precise thread geometry, correct dimensions, suitable material strength, and proper seating surfaces are essential because damaged threads, incorrect installation, insufficient tightening, or deterioration of sealing interfaces can lead to loosening, leakage, or reduced reliability of the associated components. During operation, the K 43591 screw plug may be exposed to repeated pressure fluctuations, vibration, temperature variations, and mechanical loading, requiring resistance to corrosion, deformation, and fatigue. During maintenance or overhaul, the screw plug should be inspected for thread wear, cracks, corrosion, damaged seating surfaces, deformation, and signs of loosening or leakage, while the corresponding threaded opening and any associated sealing elements should also be checked. Installation and replacement should follow the manufacturer's specifications, including the required tightening and sealing procedures. By providing secure threaded closure for part K 43553, the K 43591 screw plug contributes to the integrity of the automatic starting-air stop valve assembly, reduced risk of compressed-air leakage, reliable starting-air control, and dependable long-term operation of the ZV40/48 engine.