AccusizeTools - 4''/100mm R8 Precision Lathe Chuck, 3-Jaw Self Centering Scroll, 0225-0237 Diameter: 4'' ( 100 mm ) The body hardness is HBW 187-220. The Jaw hardness is HRC 53 - HRC 55 Works with any R8 tooling Eliminates tearing down a collet setup on a lathe Body is made of cast Iron, Jaws are made of steel.