A Grade 5 nylon lock nut, also known as a nyloc nut, is a type of fastener that uses a nylon insert to create a locking mechanism that resists loosening due to vibration or torque. The nylon insert deforms elastically around the bolt threads, creating friction and preventing the nut from backing off. Grade 5 refers to the material strength of the nut, typically made of medium carbon steel, and is a common grade found in automotive applications.