Gunsmith Screw Thread Gauge and Jig for: 8-40, 6-40, 6-48 Thread Sizes. Thread gauge and jig for the three most popular gunsmith screw thread sizes. Also doubles as a shortening jig to grind a screw to a shorter length. The thread gauge / jig is made from 1/8" steel plate, and measures 3 3/4" long X 3/8" wide.