STI-UNF V-TapGuide handles 9 Unified Fine Thread sizes (9 holes): 5/8-18, 9/16-18, 1/2-20, 7/16-20, 3/8-24, 5/6-24, 1/4-28, 10-32, 8-36 in. The guides feature a precision ground base to assure stability and accurate perpendicular alignment of taps on flat surfaces. The 90-degree V-groove design allows perpendicular alignment on round and cornered material. Boasts 100% American-made quality you can rely on.A V-TapGuide is a tap guide that answers the call to accurately guide hand-tapped threads on the inside of a hole to accept a machine screw or bolt. If every hole is started straight, it will help prevent tap breakage and increase the tap life.