The ISO Taper Gauges, both Plug and Ring type are made of superior quality high Carbon Chrome alloy Steel. Heat Treated and hardened to 58-60 HRC, to provide a high degree of wear resistance. Fine lapped to get maximum seating and surface finish. The Taper Gauges are rough ground and kept for a long period of time before finishing is given; this is done to prevent any distortions and ensure stability through their use.