STI Tap for Thread Insert Helisert STI taps are designed specifically to create internal threads that will accommodate a Coil Thread insert. Coil Thread STI Tap can be used in a range of materials including aluminum, magnesium, steel, plastics, and other free machining materials. The Spiral Tap is manufactured from High Speed Steel for durability and prolonged tooling life. Size& Type: Metric: M2-M30, for Straight and Spiral type Also from UNC2-56 to 1” Other sizes on request