Holds workpieces on the ID Tightening the hex screw expands the jaws outward and downward, preventing workpiece lift. Because the clamp expands and wedges against the I.D. of the workpiece, they are ideal for parts were there may be variations in the inside diameter of the part such as cast iron or rough finish holes. The contact can mar the surface of the workpiece, depending on the material. Using these clamps for accurately finished holes is not recommended. CP131-A uses a countersunk head screw. CP131-B uses a socket head cap screw. The jaw is made from SNCM439 steel, HRC33-39, with a black oxide finish. The washer is made from 4135 steel with a black oxide finish. The O-ring made from fluoro rubber.