316 stainless steel hex nut with a nylon-filled collar at its back end. When a screw reaches the collar, the threads and nylon form a tight, frictional fit, restricting movement of the screw when it is subjected to vibration. Designed to be used with like-material machine screws and bolts. It is able to be reused more times than a two-way reversible nut, however, nylon will begin to deteriorate at temperatures from 150? - 350?. 18-8 stainless steel is best for applications where corrosion resistance is required.Star Stainless Screw Co.