Features Specifications:Rectangular Gage Block Sets  * Steel sets are produced from a special material featuring high carbon and chrome content * Stress relieved and heat treated to 65 RC; extremely wear-resistant * Mirror-polished surface finish provides exceptional wringing characteristics * Individual serial numbers on each block, regardless of grade * Manufactured a temperature-controlled environment to provide precise and reliable accuracy * All blocks meet or exceed federal specification GGG-G-15C for flatness, parallelism and size