Alpha HSS Tap UNF Bottoming 5/8x18 UNFHB585 8x18 BOTTOMING taps have a lead of two threads and are used when tapping blind holes. Usually preceded by a taper or intermediate tap. Alpha bottoming taps can be identified by their plain shaft, they have no rings on the shaft near the driving square. Diameter: 5 8mm Pitch: 18mm Overall length: 70mm Shaft Diameter: 12mm Drive Square: 9mm Tapping Drill Size: 14. 50mm Weight (gr) 90. 00