Speedbor Drill Bit Extensions, adds 6 inches to regular bit length and follow 5/8-inch or larger bit through the hole, for all wood bits with 1/4-inch round shanks, has three flats on shank. Forged of finest quality Irwin tool steel and made in heavy-duty, double set screw type. Irwin tools are made of only the finest materials, manufactured to exacting tolerances.