The feather key, code 100100, is a fundamental mechanical element used in the MAN L21/31 engine to transmit torque between a shaft and a mounted component such as a gear, coupling, or pulley. Precisely machined to fit into matching keyways, it ensures secure alignment while allowing controlled axial movement when required. Manufactured from high-strength, wear-resistant materials, the feather key is designed to withstand repeated load cycles, vibration, and thermal influences without loss of integrity. Its simple yet critical function supports reliable power transmission, protects mating components from slippage, and contributes to the overall efficiency and durability of the engine’s drive systems.