Cap nuts, or acorn nuts, are hex nuts with a domed top.
Cap Nut, 1/2"-13, Steel, Zinc Plated, 5 PK Size: 1/2 inch-13Domed top covers and protects threadsCreates a finished appearanceAlso known as an acorn nutExposed threads should not be longer than inside height of cap nutThread Type: Coarse, Thread Class: UNC-2BSpecification(s) Met: ASME B1. Since protruding bolt tips can catch skin or clothing, use of cap nuts can also help to prevent injury..1 They cover the end of the bolt tip, which protects the threads and provides a finished appearance.