20 pack #11100, Flap disc use diagonal layers of aluminum oxide resin fiber, as it wears new grit is continuously being exposed for consistent performance throughout the life of the disc. Flap disc's are designed to run cooler to reduce warping of thin metal like body panels plus, unlike ordinary grinding wheels these discs won't gouge your work. Fits both air and electric grinders.Features: 4-1/2" Diameter Flat Type #27 Arbor Size: 7/8" Aluminum Oxide Material Fiberglass Backing Plate Maximum Speed: 13,300 rpm 80 Grit