Wilwood's HD 36 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 .001" run-out, flatness, and parallelism. Our HD Series preparation includes full detail machining and smooth pad contact faces without slots.