A Grade 10.9 nylon lock nut, also known as a nyloc nut or elastic stop nut, is a type of hex nut that utilizes a nylon insert to prevent loosening due to vibration. The nylon collar, when engaged with the bolt threads, creates a frictional fit that resists turning and effectively locks the nut in place. Grade 10.9 indicates a high-strength, heat-treated alloy steel, comparable to Grade 8 in the imperial system.