Starrett Yankee Spring-Type Inside Caliper Features include... Size: 6" / 150mm Spring Type Joint Flat Leg, Solid Nut Starrett Yankee Calipers and Dividers are made from a high-grade steel and well-finished. The bow spring, although flexible, is exceedingly strong to assure reliability. The Starrett quick-adjusting automatic-closing spring nut is designed for making fast, positive adjustments. Fulcrum stud is hardened and has a smooth bearing surface. The legs are made of flat stock and are very durable. The threads of the nut firmly engage the screw at the slightest pressure from the leg. When the pressure is withdrawn, the nut automatically releases itself, sliding freely over the screw