Ultra Series accessory pulleys are CNC-machined from lightweight 6061-T6 aluminum billet. Premium 6061-T6 aluminum billet ensures the consistency of the material's strength and balance. Tolerances for each accessory pulley is held to within .001". In addition to the numerous machining steps taken to ensure each accessory pulley is lighter than the factory piece it replaces and includes lightening holes where possible. Our aluminum accessory pulleys are also an excellent replacement for the brittle plastic pulleys found on many late model cars. Premium fit and finish ensures a beautifully matched set you are proud to display under the hood of your vehicle.