This castellated nut secures the threshing drum or beater shaft within the threshing unit of a combine harvester. It threads onto the shaft end and is locked in place with a cotter pin through one of its castellations, preventing the shaft from shifting axially under load. A loose or incorrectly torqued nut allows the threshing drum to move on its bearings, leading to vibration, uneven wear of the concave and rasp bars, and potential bearing failure. This original part is machined to the same thread pitch and fit as the factory assembly, ensuring the shaft end is properly retained without the need for shims or adapters. On reinstallation, clean the shaft threads and torque the nut to the specification for the host machine.