The Fowler 53-672-081 Steel Rectangular Economy Gage Block Set is identical to our standard sets in all respects, except for the broader range of tolerance which is more than adequate for most shop needs. These blocks are rectangular and are made of the finest grade high carbon / chrome steel that is fully stress relieved and hardened to 65 RC, lapped, and polished to a very low micro-surface finish. These blocks feature a low coefficient of thermal expansion, and they are carefully inspected for accurate size, parallelism, and flatness.