This IRWIN SPEEDBOR MAX 12" Shaft Extension features a manual locking design, which securely holds 7/16-inch hex bits. SPEEDBOR MAX bits feature 3 durable cutting edges that are resharpenable to extend the life of the drill bit. The aggressive screw pitch pulls the bit fast for clean, true holes, while the 7/16-inch impact shank helps reduce slipping in standard chucks.