2 1/2 12 UN 2A 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 2 1/2A & 3A is based on your requirements. a 2 1/2A class Ring will have a smaller (tighter) pitch diameter on both the Go and the No/Go members. Certificate of Accuracy is included.Click HERE to read about additional Long Form Cert.