1 5/16 20 UNJ 3A STEEL Go & NoGo Ring Set tool steel cold stabilized thread ring Rc 60-62 marked with size and tolerance Certificate of Accuracy is included NOTE: selection of a 2A & 3A is based on your requirements. A 2A class Ring will have a smaller (tighter) pitch diameter on both the Go and the No/Go members. A J is a special call out requiring a rounded Root Radius on 3A & 2A Go members and on some sizes both Go & NoGo members are altered to meet specification. (mil-S-8879 Aerospace Military & Government) Certificate of Accuracy is included.Click HERE to read about additional Long Form Cert.