A nyloc nut, also referred to as a nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut, is a kind of locknut.
It is a nut with a nylon collar insert that resists turning.
The plastic insert is placed at the end of the nut, and its inner diameter (ID) is slightly smaller than the major diameter of the screw. The insert deforms elastically over the threads of the screw, but threads are not cut into the nylon.
The nylon insert locks the nut by friction against the screw under the applied radial compressive force. Nyloc nuts retain their locking ability up to 250 °F (121 °C). Authorities disagree on whether nyloc nuts should be reused.
For example, Carroll Smith (Carroll Smith's Nuts, Bolts, Fasteners, and Plumbing Handbook) notes that the nylon insert is not damaged by installation and therefore they can be reused many times ,and a Federal Aviation Administration Advisory Circular allows nuts to be reused if the prevailing torque is within specification. However, an Air Force Technical Order requires replacement of self-locking nuts in critical areas.