5/16-18 Bottoming Tap for threaded insert installation. After drilling a 9.5mm deep hole with an "F" drill (.257" diameter), the tap is used to cut threads all the way to the bottom of the hole to install the insert. A standard tap won't cut threads all the way to the bottom, and not all bottoming taps cut as deeply. This one is easier to tap straight because it has four flutes, is designed for cutting wood or metal, and cuts all the way to the bottom of the hole. Fits 1/4" tap handles.