Helical coil thread repair inserts: the drill for the STI tap, the tapped hole’s pitch diameter, and the insert length at 1D to 3D, for inch and metric threads.
A thread repair insert (helical coil, often called a Heli-Coil) puts a new thread of the original size into a stripped or worn hole. The hole is drilled out, tapped with an STI tap (screw thread insert tap: the same pitch as the screw, a larger diameter), and the coil is wound in. The insert’s length is a multiple of the screw’s diameter: 1D, 1.5D, 2D, 2.5D or 3D.
An installed insert: D is the screw size it gives back, L its length. Not to scale.
| Thread | Series | STI drill | Drill (in) | Pitch dia. 3B min (in) | Pitch dia. 3B max (in) |
|---|---|---|---|---|---|
| #2-56 | UNC | — | — | 0.0976 | — |
| #3-56 | UNF | — | — | 0.1106 | 0.1119 |
| #3-48 | UNC | — | — | — | — |
| #4-48 | UNF | — | — | — | — |
| #4-40 | UNC | #31 | 0.1200″ | — | — |
| #5-40 | UNC | — | — | — | — |
| #6-40 | UNF | — | — | — | — |
| #6-32 | UNC | — | — | 0.1583 | 0.1601 |
| #8-36 | UNF | — | — | — | — |
| #8-32 | UNC | — | — | 0.1843 | 0.1862 |
| #10-32 | UNF | — | — | 0.2103 | 0.2123 |
| #10-24 | UNC | — | — | — | — |
| #12-24 | UNC | — | — | — | — |
| 1/4-28 | UNF | — | — | 0.2732 | 0.2754 |
| 1/4-20 | UNC | — | — | 0.2825 | 0.2851 |
| 5/16-24 | UNF | 21/64″ | 0.3281″ | 0.3395 | 0.3421 |
| 5/16-18 | UNC | — | — | 0.3486 | 0.3515 |
| 3/8-24 | UNF | 25/64″ | 0.3906″ | 0.4020 | 0.4047 |
| 3/8-16 | UNC | — | — | 0.4156 | 0.4189 |
| 7/16-20 | UNF | 29/64″ | 0.4531″ | 0.4700 | 0.4731 |
| 7/16-14 | UNC | 29/64″ | 0.4531″ | 0.4839 | 0.4875 |
| 1/2-20 | UNF | 33/64″ | 0.5156″ | 0.5325 | 0.5357 |
| 1/2-13 | UNC | — | — | 0.5499 | 0.5537 |
| 9/16-18 | UNF | 37/64″ | 0.5781″ | 0.5986 | 0.6020 |
| 9/16-12 | UNC | — | — | 0.6167 | 0.6208 |
| 5/8-18 | UNF | 41/64″ | 0.6406″ | 0.6611 | 0.6646 |
| 5/8-11 | UNC | 21/32″ | 0.6562″ | 0.6841 | 0.6885 |
| 3/4-16 | UNF | 49/64″ | 0.7656″ | 0.7906 | 0.7945 |
| 3/4-10 | UNC | 25/32″ | 0.7812″ | 0.8149 | 0.8196 |
| 7/8-14 | UNF | 57/64″ | 0.8906″ | 0.9214 | 0.9257 |
| 7/8-9 | UNC | 29/32″ | 0.9062″ | 0.9471 | 0.9522 |
| 1-14 | UNF | — | — | — | — |
| 1-12 | UNF | — | — | 1.0542 | 1.0589 |
| 1-8 | UNC | 1-1/32″ | 1.0312″ | 1.0812 | 1.0868 |
| 1-1/8-12 | UNF | — | — | 1.1792 | 1.1841 |
| 1-1/8-8 | 8UN | — | — | — | — |
| 1-1/8-7 | UNC | — | — | 1.2178 | 1.2239 |
| 1-1/4-12 | UNF | — | — | 1.3042 | 1.3092 |
| 1-1/4-8 | 8UN | — | — | — | — |
| 1-1/4-7 | UNC | — | — | 1.3428 | 1.3490 |
| 1-3/8-12 | UNF | — | — | 1.4292 | 1.4343 |
| 1-3/8-8 | 8UN | — | — | — | — |
| 1-3/8-6 | UNC | — | — | 1.4832 | 1.4900 |
| 1-1/2-12 | UNF | — | — | 1.5542 | 1.5595 |
| 1-1/2-8 | 8UN | — | — | — | — |
| 1-1/2-6 | UNC | — | — | 1.6082 | 1.6151 |
| 1-5/8-8 | 8UN | — | — | — | — |
| 1-3/4-8 | 8UN | — | — | — | — |
| 1-7/8-8 | 8UN | — | — | — | — |
| 2-8 | 8UN | — | — | — | — |
STI drill: printed only where every drill Heli-Coil names for the thread (its kit drill and its aluminum and steel table drills) and the drill on Recoil’s tapped-hole table are the same. Drill decimal from our drill size chart (fractions computed). Pitch diameter of the STI-tapped hole, class 3B, where both makers print the same limit. A dash: the two makers differ, or only one lists it.
| Thread | 1D | 1.5D | 2D | 2.5D | 3D |
|---|---|---|---|---|---|
| #2-56 | .086 | .129 | .172 | .215 | .258 |
| #3-56 | .099 | .149 | .198 | .247 | .297 |
| #3-48 | .099 | .149 | .198 | .247 | .297 |
| #4-48 | .112 | .168 | .224 | .280 | .336 |
| #4-40 | .112 | .168 | .224 | .280 | .336 |
| #5-40 | .125 | .188 | .250 | .312 | .375 |
| #6-40 | .138 | .207 | .276 | .345 | .414 |
| #6-32 | .138 | .207 | .276 | .345 | .414 |
| #8-36 | .164 | .246 | .328 | .410 | .492 |
| #8-32 | .164 | .246 | .328 | .410 | .492 |
| #10-32 | .190 | .285 | .380 | .475 | .570 |
| #10-24 | .190 | .285 | .380 | .475 | .570 |
| #12-24 | .216 | — | .432 | .540 | .648 |
| 1/4-28 | .250 | .375 | .500 | .625 | .750 |
| 1/4-20 | .250 | .375 | .500 | .625 | .750 |
| 5/16-24 | .312 | .469 | .625 | .781 | .938 |
| 5/16-18 | .312 | .469 | .625 | .781 | .938 |
| 3/8-24 | .375 | .562 | .750 | .938 | 1.125 |
| 3/8-16 | .375 | .562 | .750 | .938 | 1.125 |
| 7/16-20 | .438 | .656 | .875 | 1.094 | 1.312 |
| 7/16-14 | .438 | .656 | .875 | 1.094 | 1.312 |
| 1/2-20 | .500 | .750 | 1.000 | 1.250 | 1.500 |
| 1/2-13 | .500 | .750 | 1.000 | 1.250 | 1.500 |
| 9/16-18 | .562 | .844 | 1.125 | 1.406 | 1.688 |
| 9/16-12 | .562 | .844 | 1.125 | 1.406 | 1.688 |
| 5/8-18 | .625 | .938 | 1.250 | 1.562 | 1.875 |
| 5/8-11 | .625 | .938 | 1.250 | 1.562 | 1.875 |
| 3/4-16 | .750 | 1.125 | 1.500 | 1.875 | 2.250 |
| 3/4-10 | .750 | 1.125 | 1.500 | 1.875 | 2.250 |
| 7/8-14 | .875 | 1.312 | 1.750 | 2.188 | 2.625 |
| 7/8-9 | .875 | 1.312 | 1.750 | 2.188 | 2.625 |
| 1-14 | 1.000 | 1.500 | 2.000 | 2.500 | 3.000 |
| 1-12 | 1.000 | 1.500 | 2.000 | 2.500 | 3.000 |
| 1-8 | 1.000 | 1.500 | 2.000 | 2.500 | 3.000 |
| 1-1/8-12 | 1.125 | 1.688 | 2.250 | 2.812 | 3.375 |
| 1-1/8-8 | 1.125 | 1.688 | 2.250 | 2.812 | 3.375 |
| 1-1/8-7 | 1.125 | 1.688 | 2.250 | 2.812 | 3.375 |
| 1-1/4-12 | 1.250 | 1.875 | 2.500 | 3.125 | 3.750 |
| 1-1/4-8 | 1.250 | 1.875 | 2.500 | 3.125 | 3.750 |
| 1-1/4-7 | 1.250 | 1.875 | 2.500 | 3.125 | 3.750 |
| 1-3/8-12 | 1.375 | 2.062 | 2.750 | 3.438 | 4.125 |
| 1-3/8-8 | 1.375 | 2.062 | 2.750 | 3.438 | 4.125 |
| 1-3/8-6 | 1.375 | 2.062 | 2.750 | 3.438 | 4.125 |
| 1-1/2-12 | 1.500 | 2.250 | 3.000 | 3.750 | 4.500 |
| 1-1/2-8 | 1.500 | 2.250 | 3.000 | 3.750 | 4.500 |
| 1-1/2-6 | 1.500 | 2.250 | 3.000 | 3.750 | 4.500 |
| 1-5/8-8 | 1.625 | 2.438 | 3.250 | 4.062 | 4.875 |
| 1-3/4-8 | 1.750 | 2.625 | 3.500 | 4.375 | 5.250 |
| 1-7/8-8 | 1.875 | 2.812 | 3.750 | 4.688 | 5.625 |
| 2-8 | 2.000 | 3.000 | 4.000 | 5.000 | 6.000 |
Nominal insert length in inches: the multiple times the screw’s nominal diameter (computed, to 3 places). A dash: Heli-Coil lists a different length for that size.
| Thread | STI drill | Drill (in) | Pitch dia. min (mm) |
|---|---|---|---|
| M2x0.4 | 2.1 mm | 0.0827″ | 2.260 |
| M2.2x0.45 | — | — | 2.492 |
| M2.5x0.45 | — | — | 2.792 |
| M3x0.5 | — | — | 3.325 |
| M3.5x0.6 | — | — | 3.890 |
| M4x0.7 | — | — | 4.455 |
| M5x0.8 | — | — | 5.520 |
| M6x1 | — | — | 6.650 |
| M7x1 | — | — | 7.650 |
| M8x1 | — | — | 8.650 |
| M8x1.25 | — | — | — |
| M9x1.25 | — | — | — |
| M10x1 | — | — | — |
| M10x1.25 | — | — | 10.812 |
| M10x1.5 | — | — | 10.974 |
| M11x1.5 | — | — | — |
| M12x1.25 | — | — | 12.812 |
| M12x1.5 | — | — | — |
| M12x1.75 | — | — | 13.137 |
| M14x1.5 | — | — | 14.974 |
| M14x2 | — | — | 15.299 |
| M16x1.5 | — | — | 16.974 |
| M16x2 | — | — | 17.299 |
| M18x1.5 | — | — | 18.974 |
| M18x2 | 18.5 mm | 0.7283″ | 19.299 |
| M18x2.5 | — | — | 19.624 |
| M20x1.5 | — | — | 20.974 |
| M20x2 | 20.5 mm | 0.8071″ | 21.299 |
| M20x2.5 | — | — | 21.624 |
| M22x1.5 | — | — | 22.974 |
| M22x2 | 22.5 mm | 0.8858″ | 23.299 |
| M22x2.5 | — | — | 23.624 |
| M24x1.5 | — | — | — |
| M24x2 | — | — | 25.299 |
| M24x3 | 24.75 mm | 0.9744″ | — |
| M27x2 | — | — | — |
| M27x3 | — | — | — |
| M30x2 | — | — | — |
| M30x3.5 | — | — | 32.273 |
| M33x2 | — | — | — |
| M33x3.5 | — | — | 35.273 |
| M36x2 | — | — | — |
| M36x3 | — | — | — |
| M36x4 | — | — | 38.598 |
| M39x2 | — | — | — |
| M39x3 | — | — | — |
| M39x4 | — | — | 41.598 |
Heli-Coil’s metric kits carry inch drills and Recoil lists mm drills, so most metric drills show a dash: use the drill packed in the kit or the maker’s chart. Inch of a mm drill computed from 25.4 mm = 1 inch. Pitch diameter minimum of the STI-tapped hole where both makers print the same value (the maximum depends on the tolerance class, which the two makers name differently).
| Thread | 1D | 1.5D | 2D | 2.5D | 3D |
|---|---|---|---|---|---|
| M2x0.4 | 2 | 3 | 4 | 5 | 6 |
| M2.2x0.45 | 2.2 | 3.3 | 4.4 | 5.5 | 6.6 |
| M2.5x0.45 | 2.5 | 3.75 | 5 | 6.25 | 7.5 |
| M3x0.5 | 3 | 4.5 | 6 | 7.5 | 9 |
| M3.5x0.6 | 3.5 | 5.25 | 7 | 8.75 | 10.5 |
| M4x0.7 | 4 | 6 | 8 | 10 | 12 |
| M5x0.8 | 5 | 7.5 | 10 | 12.5 | 15 |
| M6x1 | 6 | 9 | 12 | 15 | 18 |
| M7x1 | 7 | 10.5 | 14 | 17.5 | 21 |
| M8x1 | 8 | 12 | 16 | 20 | 24 |
| M8x1.25 | 8 | 12 | 16 | 20 | 24 |
| M9x1.25 | 9 | 13.5 | 18 | 22.5 | 27 |
| M10x1 | 10 | 15 | 20 | 25 | 30 |
| M10x1.25 | 10 | 15 | 20 | 25 | 30 |
| M10x1.5 | 10 | 15 | 20 | 25 | 30 |
| M11x1.5 | 11 | 16.5 | 22 | 27.5 | 33 |
| M12x1.25 | 12 | 18 | 24 | 30 | 36 |
| M12x1.5 | 12 | 18 | 24 | 30 | 36 |
| M12x1.75 | 12 | 18 | 24 | 30 | 36 |
| M14x1.5 | 14 | 21 | 28 | 35 | 42 |
| M14x2 | 14 | 21 | 28 | 35 | 42 |
| M16x1.5 | 16 | 24 | 32 | 40 | 48 |
| M16x2 | 16 | 24 | 32 | 40 | 48 |
| M18x1.5 | 18 | 27 | 36 | 45 | 54 |
| M18x2 | 18 | 27 | 36 | 45 | 54 |
| M18x2.5 | 18 | 27 | 36 | 45 | 54 |
| M20x1.5 | 20 | 30 | 40 | 50 | 60 |
| M20x2 | 20 | 30 | 40 | 50 | 60 |
| M20x2.5 | 20 | 30 | 40 | 50 | 60 |
| M22x1.5 | 22 | 33 | 44 | 55 | 66 |
| M22x2 | 22 | 33 | 44 | 55 | 66 |
| M22x2.5 | 22 | 33 | 44 | 55 | 66 |
| M24x1.5 | 24 | 36 | 48 | 60 | 72 |
| M24x2 | 24 | 36 | 48 | 60 | 72 |
| M24x3 | 24 | 36 | 48 | 60 | 72 |
| M27x2 | 27 | 40.5 | 54 | 67.5 | 81 |
| M27x3 | 27 | 40.5 | 54 | 67.5 | 81 |
| M30x2 | 30 | 45 | 60 | 75 | 90 |
| M30x3.5 | 30 | 45 | 60 | 75 | 90 |
| M33x2 | 33 | 49.5 | 66 | 82.5 | 99 |
| M33x3.5 | 33 | 49.5 | 66 | 82.5 | 99 |
| M36x2 | 36 | 54 | 72 | 90 | 108 |
| M36x3 | 36 | 54 | 72 | 90 | 108 |
| M36x4 | 36 | 54 | 72 | 90 | 108 |
| M39x2 | 39 | 58.5 | 78 | 97.5 | 117 |
| M39x3 | 39 | 58.5 | 78 | 97.5 | 117 |
| M39x4 | 39 | 58.5 | 78 | 97.5 | 117 |
Nominal insert length in mm: the multiple times the nominal diameter (computed). A dash: Heli-Coil lists a different length for that size. M24x1.5: Heli-Coil does not list it; its lengths are where V-COIL and Recoil agree (drill and pitch diameter: a dash).
Good to know
Use the STI tap, not a regular tap. The insert is bigger than the hole until it is wound in, so the hole needs the STI tap for the same thread. Recoil: “They have a larger diameter but the same pitch as a standard tap in order to accommodate the wire insert.” Heli-Coil: “Heli-Coil inserts are larger in diameter before installation than the tapped hole.”
Wind it in below the surface. Heli-Coil: “Wind the insert into the hole until the top coil is 1/4 to 1/2 turn below the top surface.” Recoil: “Wind insert in with light downward pressure until 1/4 to 1/2 turn below the surface.” Then break off the tang (the bent end that drives it).
Check the tapped hole with an STI thread plug gauge. It checks the pitch diameter of the tapped hole; if the hole gauges right, the installed insert gives the screw’s own thread.
A dash on the drill means our maker sources don’t all name the same drill (Heli-Coil’s kit and table drills differ for many sizes): use the drill in the kit, or the maker’s chart.
Quick answers
- What drill size do I use for a thread repair insert (Helicoil)?
- Where every Heli-Coil and Recoil drill agrees: 7/16-14 29/64″, 5/8-11 21/32″, 3/4-10 25/32″, 3/8-24 25/64″, then the STI tap. For sizes such as 1/4-20 the makers’ charts name more than one drill — use the drill packed in the kit.
- Can I use a regular tap for a thread repair insert?
- No. The insert needs an STI (screw thread insert) tap: same pitch as the screw but a larger diameter, so the coil fits in it. Heli-Coil and Recoil both say to use their STI taps.
- What does 1.5D mean on a thread repair insert?
- The insert’s length is 1.5 times the screw’s diameter. A 1/2-13 x 1.5D insert is .750″ long; a 1/4-20 x 2D is .500″.
- What drill for an M8 x 1.25 thread repair insert?
- Our two maker sources name different drills for most metric sizes, so the chart shows a dash: use the drill packed in the kit, or the insert maker’s own chart.
- How far down does a thread repair insert go?
- Wind it in until the top coil is 1/4 to 1/2 turn below the surface, then break off the tang (both makers).