Helicoil / STI tap drill chart
To install a wire thread insert (Helicoil-type), drill with the STI drill, tap with the matching STI tap, then install the insert. Insert lengths are listed in diameters (1D, 1.5D, 2D). Follow the insert maker's chart for the exact drill and tap.
| Thread | Insert | STI drill | Decimal (in) |
|---|---|---|---|
| #1-64 | #1-64×1D | #43 | 0.0890 |
| #2-56 | #2-56×1D | #41 | 0.0960 |
| #3-48 | #3-48×1D | #37 | 0.1040 |
| #4-40 | #4-40×1D | #31 | 0.1200 |
| #5-40 | #5-40×1D | #29 | 0.1360 |
| #6-32 | #6-32×1D | #25 | 0.1495 |
| #8-32 | #8-32×1D | #18 | 0.1695 |
| #10-24 | #10-24×1D | #10 | 0.1935 |
| 1/4-20 | 1/4-20×1D | F | 0.2570 |
| 5/16-18 | 5/16-18×1D | Q | 0.3320 |
| 3/8-16 | 3/8-16×1D | 25/64" | 0.3906 |
| 7/16-14 | 7/16-14×1D | 29/64" | 0.4531 |
| 1/2-13 | 1/2-13×1D | 33/64" | 0.5156 |
| 5/8-11 | 5/8-11×1D | 41/64" | 0.6406 |
| 3/4-10 | 3/4-10×1D | 49/64" | 0.7656 |
| 7/8-9 | 7/8-9×1D | 57/64" | 0.8906 |
| 1"-8 | 1"-8×1D | 1-1/64" | 1.0156 |
| Thread | Insert | STI drill | Decimal (in) |
|---|---|---|---|
| #0-80 | #0-80×1D | #49 | 0.0730 |
| #1-72 | #1-72×1D | #45 | 0.0820 |
| #2-64 | #2-64×1D | #42 | 0.0935 |
| #4-48 | #4-48×1D | #27 | 0.1440 |
| #6-40 | #6-40×1D | #19 | 0.1660 |
| #8-36 | #8-36×1D | #13 | 0.1850 |
| #10-32 | #10-32×1D | #3 | 0.2130 |
| 1/4-28 | 1/4-28×1D | H | 0.2660 |
| 5/16-24 | 5/16-24×1D | S | 0.3480 |
| 3/8-24 | 3/8-24×1D | 27/64" | 0.4219 |
| 7/16-20 | 7/16-20×1D | 31/64" | 0.4844 |
| 1/2-20 | 1/2-20×1D | 35/64" | 0.5469 |
| 9/16-18 | 9/16-18×1D | 39/64" | 0.6094 |
| 3/4-16 | 3/4-16×1D | 53/64" | 0.8281 |
| Thread | Insert | STI drill | Decimal (mm) |
|---|---|---|---|
| M3×0.5 | M3×0.5×1D | 3.4mm | 3.40 |
| M4×0.7 | M4×0.7×1D | 4.5mm | 4.50 |
| M5×0.8 | M5×0.8×1D | 5.6mm | 5.60 |
| M6×1.0 | M6×1.0×1D | 6.7mm | 6.70 |
| M8×1.25 | M8×1.25×1D | 9.0mm | 9.00 |
| M10×1.5 | M10×1.5×1D | 10.7mm | 10.70 |
| M12×1.75 | M12×1.75×1D | 12.8mm | 12.80 |
| M14×2.0 | M14×2.0×1D | 15.0mm | 15.00 |
| M16×2.0 | M16×2.0×1D | 17.0mm | 17.00 |
| M20×2.5 | M20×2.5×1D | 21.5mm | 21.50 |
| M24×3.0 | M24×3.0×1D | 25.5mm | 25.50 |
Common questions
- What size drill for a 1/4-20 Helicoil?
- Our data lists an F drill (0.257 in), then the 1/4-20 STI tap. Some insert makers specify 17/64 in — follow the insert maker’s chart.
- What does STI mean?
- Screw Thread Insert. The STI tap is oversized so the installed insert ends up at the nominal thread size.
- Can I use a regular tap for a Helicoil?
- No. The hole must be tapped with an STI tap of the same nominal size and pitch.
Calculators that use this data
Tap Drill & Thread % CalculatorPercent of thread for any drill (cut and form taps), 75 / 65 / 50% tap drills, UNC / UNF / metric chart with thread limits, every thread to 4 in and M64, pipe taps and STI / Helicoil.Drilling CalculationsDrilling RPM, feed per revolution, point allowance and cutting time.
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