Range: 5/16" to 6". Double action rigid handle with self-centering feature for quick checking of inside measurements. Constant spring tension for uniform feel. Contact point ends are hardened tool steel and precision ground on a radius of the smallest hole the gage will enter. Covers a measuring range of 5/16" to 6" with measurable depths of 4" to 5" Another great tool for easy and accurate measurements. Under spring tension, the plungers of the gage expand from the center to the size of the hole being measured and are then locked in place using the knurled knob. Final hole size is obtained by measuring over the contacts with a micrometer or vernier caliper.