Wilwood HD 48 Curved Vane Spec-37 rotors are manufactured from a proprietary iron alloy developed to withstand extreme temperatures with the highest possible degree of resistance against distortion warping cracking and wear. The formulation for this alloy is a derivative of technology and materials that were significant in the development of the extreme duty military spec rotors that are manufactured by Wilwood. Every Spec 37 rotor is precision machined to less than .001IN run-out flatness and parallelism. Our HD Series preparation includes full detail machining and smooth pad contact faces without slots. Wilwood Ultra-Light straight vane rotors provide high value performance and rugged durability for a wide range of competition high performance and sport driving applications. Dont confuse these rotors with bargain priced off-brand named parts. Wilwoods modern manufacturing capability combined with large scale econmies make it possible to offer this level of quality and performance at such an affordable price.