Cap Nuts are usually made of brass, steel, stainless steel (low carbon content) or nylon. They can also be chrome plated and given a mirror finish.
There are two types of cap nuts. One is low, or the standard cap nut. The other is the high cap nut. The high cap nut is wider and higher and will protect extra long studs. There are also self-locking cap nuts that have distorted threads in the hex area to create a tight friction fit to prevent the nut from vibrating loose.
There are standards governing the manufacture of cap nuts. One is Society of Automotive Engineers (SAE) Standard J483, High and Low Crown (Blind, cap) Hex Nuts. Another is Deutsches Institut für Normung (DIN) 1587, Hexagon Domed Cap Nuts.
cap nut, dome nut, domed cap nut
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