Inside spring calipers are used for measuring and transferring precise inside (ID) measurements. These calipers are made from high-grade carbon steel with hardened, rounded and finely polished points. They feature a quick-spring nut for rapid and precise fine adjustments that will hold any measurement. Adjustment mechanism is central located to assure smooth action. Fulcrum and adjustment mechanism are brass. “Yankee” style flat-leg construction makes unit very durable. They have a substantially hardened spring for years of service. Maximum opening is 12 inches or 300 mm.