Mild steel castle nuts (or castellated nuts) are hex nuts with a rounded, slotted, crown-like turret, designed for high-vibration applications requiring a cotter pin for positive locking.
Often grade 4 mild steel, they prevent loosening on axles and hubs.
Their design differs from slotted nuts by having a turned-down turret that allows the cotter pin to wrap around closer to the nut, providing added security
Pitch 5.0mm
