he Threaded Stub is used in the drilling end of the Turbo Tunnel Bit and acts as a bit guide/center punch until the carbide teeth of the Turbo Tunnel bit grab into the concrete being drilled. FEATURES: 2-1/2 in. length threaded stud. Interchangeable drilling heads. SDS-max or spline shanks available. 11 in. extensions, 48-20-5079, can be coupled for extra deep drilling applications