Thread insert drill chart
Drill sizes and STI tap callouts for helical coil thread inserts in common UNC, UNF and metric sizes, with standard insert lengths.
Thread insert drill chart
Helical coil inserts for aluminum, magnesium, plastics and thread repair. Drill size for the STI tap, insert length and how deep to drill a blind hole.
The STI tap cuts an oversize thread the wire coil nests into, so the drill is larger than a normal tap drill. Sizes shown match the common helical insert charts.
Insert length is sold as a multiple of the nominal diameter: 1D for steel, 1.5D for aluminum and cast iron, 2D or more for plastics and soft castings. Installed length L = n · d.
Tap depth = L + (extra threads) · pitch, so the insert sits a turn or two below the surface. Drill depth adds the drill point and a little for the tap chamfer. Deeper is fine, shallower strips the insert.
Confirm sizes against the chart that came with your insert kit. Use a bottoming STI tap for blind holes. Break the tang after installation and check the insert sits one quarter to one half turn below the surface.
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