The locking spring washer, code 515151, is a resilient fastening component used in the MAN L21/31 engine to help secure bolted connections against loosening caused by vibration, cyclic loads, thermal expansion, and other dynamic operating influences. Its spring characteristics allow it to exert an elastic reaction within the fastening arrangement, helping maintain contact and supporting the required clamping effect when the connection is correctly assembled. Because medium-speed diesel engines operate under continuous vibration and changing mechanical and thermal loads, the condition, elasticity, dimensions, and material strength of such a washer are important for dependable fastening performance. Correct positioning and tightening according to the specified assembly procedure are essential, while damaged, permanently deformed, cracked, heavily corroded, or fatigued washers should not be relied upon to provide their intended securing function. By supporting the stability of critical bolted connections, the locking spring washer helps reduce the possibility of unintended loosening, improves mechanical security, and contributes to the safe and reliable long-term operation of the MAN L21/31 engine.