# Helical coil thread inserts, inch UNC and UNF (Heli-Coil style): drill size, tapped hole, depth for 1D to 3D inserts, #1 to 2 in

> What drill size and tap does a 1/4-20 or 5/16-18 helical coil thread repair insert need, and how deep must the hole be?

- Category: Hardware › Fasteners
- Kind: Standard part
- Basis: Misumi's inch Helicoil tap and drill chart (UNC #1 to 2 in, UNF #2 to 2 in); a distributor's STI sizing chart read as a check and not used where its drill sizes differ; numbers only
- Page: https://xemstudio.com/objects/helical-thread-inserts-inch-unc-unf/ · JSON: https://xemstudio.com/api/geometry?object=helical-thread-inserts-inch-unc-unf · Plain text: https://xemstudio.com/objects/helical-thread-inserts-inch-unc-unf/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files CC BY 4.0 or their publisher's license
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**Exact dimensions.** True size, with no tolerance added.

## Notes

- Inches. The table is Misumi's: thread size (nominal and decimal) and threads per inch, the maximum major diameter the insert tap cuts (the tapped hole into which the coil screws), the drill, and the minimum tapped depth H for insert lengths 1D, 1.5D, 2D, 2.5D and 3D (D = nominal thread diameter, the usual meaning of those lengths). The depth columns are labelled in the source as the minimum tapping depth for each length; each is 0.05 in (small sizes) to 0.1 in or more above the insert length, which is consistent with a hole that is a little deeper than the insert.
- Drills are given as number, letter-free fraction or fraction exactly as Misumi prints them (no decimal equivalent is printed; none is supplied here). The metric DIN 8140 sizes are on helical-thread-inserts-din-8140 (M3 to M16).
- Check: the Albany County Fasteners STI chart (a stainless-insert system, #4-40 to 3/4-16) prints different drills for some sizes (for example #3 for 1/4-20, where Misumi's coil insert drill is 17/64) and one minimum-depth column, so it is not merged here. Use the maker's chart for the insert you buy; free-running and locking versions of the same insert use the same drill.
- No files: the coil is a helix whose wire section is not published as numbers in the table read. McMaster-Carr sells mil-spec helical inserts with per-part STEP; linked, not copied.

## Sizes

| series | thread size | TPI | tap major max (in) | drill | min depth 1D (in) | 1.5D | 2D | 2.5D | 3D |
|---|---|---|---|---|---|---|---|---|---|
| UNC | 1 (.073)-64 | 64 | .0958 | #53 | .090 | .125 | .160 | .200 | .225 |
| UNC | 2 (.086)-56 | 56 | .1117 | 3/32 | .100 | .150 | .190 | .230 | .280 |
| UNC | 3 (.099)-48 | 48 | .1289 | #36 | .120 | .170 | .220 | .270 | .320 |
| UNC | 4 (.112)-40 | 40 | .1473 | #31 | .140 | .190 | .250 | .310 | .360 |
| UNC | 5 (.125)-40 | 40 | .1603 | #29 | .150 | .210 | .280 | .340 | .400 |
| UNC | 6 (.138)-32 | 32 | .1817 | #25 | .170 | .240 | .310 | .380 | .450 |
| UNC | 8 (.164)-32 | 32 | .2077 | 11/64 | .200 | .280 | .360 | .440 | .520 |
| UNC | 10 (.190)-24 | 24 | .2475 | 13/64 | .230 | .330 | .420 | .520 | .610 |
| UNC | 12 (.216)-24 | 24 | .2735 | 15/64 | .260 | .370 | .470 | .580 | .690 |
| UNC | 1/4 (.2500)-20 | 20 | .3187 | 17/64 | .300 | .430 | .550 | .680 | .800 |
| UNC | 5/16 (.3125)-18 | 18 | .3884 | 21/64 | .370 | .530 | .680 | .840 | .990 |
| UNC | 3/8 (.3750)-16 | 16 | .4602 | 25/64 | .440 | .630 | .810 | 1.000 | 1.190 |
| UNC | 7/16 (.4375)-14 | 14 | .5343 | 29/64 | .510 | .730 | .950 | 1.170 | 1.380 |
| UNC | 1/2 (.5000)-13 | 13 | .6042 | 33/64 | .580 | .830 | 1.080 | 1.330 | 1.580 |
| UNC | 9/16 (.5625)-12 | 12 | .6751 | 37/64 | .650 | .930 | 1.210 | 1.490 | 1.770 |
| UNC | 5/8 (.6250)-11 | 11 | .7477 | 21/32 | .720 | 1.030 | 1.340 | 1.650 | 1.970 |
| UNC | 3/4 (.7500)-10 | 10 | .8850 | 25/32 | .850 | 1.230 | 1.600 | 1.980 | 2.350 |
| UNC | 7/8 (.8750)-9 | 9 | 1.0247 | 57/64 | .990 | 1.420 | 1.860 | 2.300 | 2.740 |
| UNC | 1 (1.000)-8 | 8 | 1.1681 | 1-1/32 | 1.130 | 1.630 | 2.130 | 2.630 | 3.130 |
| UNC | 1-1/8 (1.1250)-7 | 7 | 1.3171 | 1-5/32 | 1.270 | 1.830 | 2.390 | 2.960 | 3.520 |
| UNC | 1-1/4 (1.2500)-7 | 7 | 1.4421 | 1-9/32 | 1.390 | 2.020 | 2.640 | 3.270 | 3.890 |
| UNC | 1-3/8 (1.3750)-6 | 6 | 1.5982 | 1-13/32 | 1.540 | 2.230 | 2.920 | 3.600 | 4.290 |
| UNC | 1-1/2 (1.5000)-6 | 6 | 1.7232 | 1-17/32 | 1.670 | 2.420 | 3.170 | 3.920 | 4.670 |
| UNC | 1-3/4 (1.7500)-5 | 5 | 1.9922 | 1-13/16 | 1.920 | 2.790 | 3.650 | 4.520 | 5.380 |
| UNC | 2 (2.0000)-4.5 | 4.5 | 2.2611 | 2-1/16 | 2.170 | 3.160 | 4.140 | 5.120 | 6.100 |
| UNF | 2 (.086)-64 | 64 | .1088 | #55 | .100 | .145 | .190 | .230 | .275 |
| UNF | 3 (.099)-56 | 56 | .1247 | #48 | .120 | .170 | .220 | .270 | .310 |
| UNF | 4 (.112)-48 | 48 | .1419 | #43 | .130 | .190 | .240 | .300 | .360 |
| UNF | 6 (.138)-40 | 40 | .1733 | #33 | .160 | .230 | .300 | .370 | .440 |
| UNF | 8 (.164)-36 | 36 | .2032 | #29 | .190 | .270 | .360 | .440 | .520 |
| UNF | 10 (.190)-32 | 32 | .2337 | #21 | .220 | .320 | .410 | .510 | .600 |
| UNF | 1/4 (.2500)-28 | 28 | .2995 | 17/64 | .290 | .410 | .540 | .660 | .790 |
| UNF | 5/16 (.3125)-24 | 24 | .3700 | 21/64 | .350 | .510 | .670 | .820 | .980 |
| UNF | 3/8 (.3750)-24 | 24 | .4325 | 25/64 | .420 | .600 | .790 | .980 | 1.170 |
| UNF | 7/16 (.4375)-20 | 20 | .5062 | 29/64 | .490 | .710 | .930 | 1.140 | 1.360 |
| UNF | 1/2 (.5000)-20 | 20 | .5687 | 33/64 | .550 | .800 | 1.050 | 1.300 | 1.550 |
| UNF | 9/16 (.5625)-18 | 18 | .6384 | 41/64 | .620 | .900 | 1.180 | 1.460 | 1.740 |
| UNF | 5/8 (.6250)-18 | 18 | .7009 | 41/64 | .680 | .990 | 1.310 | 1.620 | 1.930 |
| UNF | 3/4 (.7500)-16 | 16 | .8352 | 49/64 | .810 | 1.190 | 1.560 | 1.940 | 2.310 |
| UNF | 7/8 (.8750)-14 | 14 | .9718 | 29/32 | .950 | 1.380 | 1.820 | 2.260 | 2.700 |
| UNF | 1 (1.000)-14 | 14 | 1.0968 | 1-1/64 | 1.070 | 1.570 | 2.070 | 2.570 | 3.070 |
| UNF | 1 (1.000)-12 | 12 | 1.1126 | 1-1/16 | 1.080 | 1.580 | 2.080 | 2.580 | 3.080 |
| UNF | 1-1/8 (1.1250)-12 | 12 | 1.2376 | 1-5/32 | 1.210 | 1.770 | 2.330 | 2.900 | 3.460 |
| UNF | 1-1/4 (1.2500)-12 | 12 | 1.3626 | 1-9/32 | 1.330 | 1.960 | 2.580 | 3.210 | 3.830 |
| UNF | 1-3/8 (1.3750)-12 | 12 | 1.4876 | 1-13/32 | 1.460 | 2.150 | 2.830 | 3.520 | 4.210 |
| UNF | 1-1/2 (1.5000)-12 | 12 | 1.6126 | 1-17/32 | 1.580 | 2.330 | 3.080 | 3.830 | 4.580 |
| UNF | 1-3/4 (1.7500)-12 | 12 | 1.8626 | 1-13/16 | 1.830 | 2.710 | 3.580 | 4.460 | 5.330 |
| UNF | 2 (2.0000)-12 | 12 | 2.1126 | 2-1/16 | 2.080 | 3.080 | 4.080 | 5.080 | 6.080 |

## Where the files are

- Misumi (US blog): [Helicoil tap and drill size chart: UNC and UNF drill, tapped-hole major, minimum depth for 1D to 3D inserts](https://us.misumi-ec.com/blog/helicoil-tap-and-drill-size-chart/) — web — Publisher's page; dimensions are facts
- Albany County Fasteners: [Thread repair sizing chart (SAE inch and metric STI): drill, STI tap, minimum hole depth (a different drill column; not used)](https://www.albanycountyfasteners.com/media/be/f8/b9/1764685703/thread-repair-sizing-chart.pdf) — web — Distributor PDF; numbers only
- McMaster-Carr: [Helical (thread-repair) inserts, mil-spec (free STEP/IGES per part, for your own design use; no redistribution)](https://www.mcmaster.com/mil-spec-helical-inserts) — STEP, IGES, PDF — McMaster-Carr site terms: personal design use, no redistribution (verify at link)

## Questions

**What drill size for a 1/4-20 Heli-Coil?**

17/64 in (UNC), tapped hole major up to 0.3187 in; minimum depth 0.300 in for a 1D insert, 0.430 in for 1.5D, 0.550 in for 2D (Misumi).

**What drill size for a 3/8-16 helical insert?**

25/64 in; tapped hole major up to 0.4602 in; minimum depth 0.440 in for 1D, 0.630 in for 1.5D, 0.810 in for 2D.

**What drill for a 10-32 Heli-Coil?**

#21 (UNF), tapped hole major up to 0.2337 in, minimum depth 0.220 in for 1D.

## Fits with

- [Helical wire thread inserts (Helicoil style, DIN 8140): M3–M16 drill size, tapped hole, insert length 1D–3D](https://xemstudio.com/objects/helical-thread-inserts-din-8140/)
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- [Threaded rod, inch: #4 to 4 in, UNC and UNF threads per inch, major diameter in inches and mm, standard lengths](https://xemstudio.com/objects/threaded-rod-inch-unc-unf-sizes/)
- [Thread size reference: UNC / UNF and ISO metric coarse / fine — major, minor and pitch diameter, tap drill, clearance hole (close, normal, loose)](https://xemstudio.com/objects/thread-reference-tap-drills-clearance/)
- [Imperial machine screws: #4-40, #6-32, #8-32, 1/4-20](https://xemstudio.com/objects/imperial-machine-screws/)
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