The Southern Adjustable Style thread Ring Gage features an adjustment mechanism that maintains the roundness integrity of the gage throughout its use. The helix path of the thread along the line of adjustment remains true and in alignment throughout the life of the gage. Aluminum gage bodies anodized green for Go and red for NoGo provide easy identification. Precision ground thread inserts are made of oil hardened tool steel. Thread class and tolerances conform to ASME/ANSI B1.2 or B1.16M. Sizes below .2500 and M6 are counterbored to hold specific length. https://www.threadcheck.com/southern-style-thread-ring-gages-unified-inch-steel/unified-inch-thread-gages/ https://www.threadcheck.com/southern-style-thread-ring-gages-metric-steel/unified-metric-thread-gages/