A castle nut, with slots (notches) cut into one end. The name comes from the nut’s resemblance to the crenellated parapet of a medieval castle. Castellated nuts are sometimes referred to incorrectly as castigated nuts.
EN 10204.3.1 (2004)
In Fact These nuts are used with bolts and studs that have a hole drilled in their threaded section through which a cotter pin is inserted to prevent the nut from rotating. The nut is quite effectively retained until the pin is removed or shears. They are a different fastener altogether. It visually look more like a hex nut and host protruding triangular portions, where as a slotted nut (pictured to the right), looks as though a hex nut has been cut with the slots and corners forming part of the protrusion. This is a slotted nut.
In brief Despite experience and lessons learned from LE armorers and users, some still do not believe it’s necessary to stake the nut. Staking the castle nut is the operation where material is moved from the end plate into the castle nut’s staking notches with a mallet and punch, rendering the nut immobile. there is no “need” for the castle nut wrench. It IS the best too for the job, and there are other specialty wrenched that also work well. But you ABSOLUTELY can install it and get it very tight with a hammer and a flat screwdriver to snug it up.