For use with an externally threaded item that measures 5/16" Inch and has 18 threads per inchPackage contains 25 piecesHeight: .4675"Hex Socket Drive 9/16"Nickel plated steel offers a nice shiny finish and good corrosion resistanceAn acorn nut, also referred to as crown hex nut, blind nut, cap nut, domed cap nut, or dome nut (UK), is a nut that has a domed end on one side. When used together with a threaded fastener with an external male thread, the domed end encloses the external thread of the threaded fastener, either to protect the thread itself or to protect nearby objects from contact with the thread. In addition, the dome gives a more finished appearance.