The Irwin Blue Groove 4X Flat Bits have a patented groove point, and cutting edge designed to cut and not scrape the timber 4 times faster than standard flat bits. The parabolic scoop optimises control and removal of wood chips. Increased relief angle from 12 to 17. The shank is 152mm in length and 10% larger in diameter for added durability. The Irwin IRW10502813 Blue Groove 4x flat bit has the following specific dimension:Diameter: 20mm IRWIN Blue Groove 4X Flat Bit 20Mm X 152Mm in Silver