Thread Diameter: 1/2" Threads per Inch: 13 High Speed Steel. Ground Thread. Recommended Drill Size: 27/64" Bottoming taps have 1-1 1/2 threads of lead, for threading close to the bottom of a blind hole. Please note that not all bottoming taps are flattened at the end. Some manufacturers leave a point that you may have to grind off if going very close to the bottom of a hole.