Nylon insert lock nuts are hex nuts that have a captive nylon washer mounted in their top to create a prevailing torque type lock nut. The undersized washer forms a compression grip that binds on the threads of the mating fastener to resist loosening. Due to limitations of the non-metallic insert, the nuts cannot be used at elevated temperatures or with certain chemicals but the nylon washer does provide a gas and moisture seal. Various colors of nylon are used. Nylon insert lock nuts are also known as:  aircraft lock nuts, elastic lock nuts, elastic stop nuts, nyloc nuts, nylock nuts, nylon insert stop nuts, stop nuts.