The Castle nuts and split pin are used together to prevent loosening. The nuts has a notch (notch) at one end. The name derives from the appearance of a nuts, which resembles the zigzag parapet of a castle. The bolt or shaft has one or two holes drilled on its threaded end. Tighten the nut appropriately, then, if the slot is not aligned with the hole in the fastener, rotate the nut to the nearest slot.