Our Derale 18″ heavy duty standard rotation fan features thick black steel hubs and 6 stainless reinforced blades with a large surface area that improve performance and fuel economy and is rated up to 10,000 rpm. The large surface area of the blades pull more air at lower speeds than other flex fans. Reinforced blade construction extends fan life. Dimensions: Diameter: 18-1/4″, Flange bolt spacing: 1-3/4″ – 2-1/4″, Depth 2-3/8″, Mounting surface to front: 1/2″, Mounting surface to rear 1-7/8″. Rotation: Standard (clockwise).