# 1/4-20 tap drill size

URL: https://www.clove3d.com/tools/tap-drill/1-4-20/

Tap drill for 1/4-20 UNC: #7 (0.2010 in / 5.11 mm), about 75% thread.

## Thread specs

- Series: UNC
- Major diameter: 0.2500 in
- Pitch: 20 TPI
- Basic minor diameter: 0.1959 in

## Every drill that works (50-85% thread)

| Drill | % thread | in | mm | Good for |
|---|---|---|---|---|
| 5.5 mm | 52% | 0.2165 | 5.50 | deep holes only |
| #3 | 57% | 0.2130 | 5.41 | deep holes only |
| 5.4 mm | 58% | 0.2126 | 5.40 | deep holes only |
| #4 | 63% | 0.2090 | 5.31 | hard or tough metals |
| 5.3 mm | 64% | 0.2087 | 5.30 | hard or tough metals |
| #5 | 69% | 0.2055 | 5.22 | hard or tough metals |
| 5.2 mm | 70% | 0.2047 | 5.20 | hard or tough metals |
| #6 | 71% | 0.2040 | 5.18 | general use |
| 13/64" | 72% | 0.2031 | 5.16 | general use |
| #7 (standard) | 75% | 0.2010 | 5.11 | general use |
| 5.1 mm | 76% | 0.2008 | 5.10 | general use |
| #8 | 79% | 0.1990 | 5.05 | general use |
| 5.0 mm | 82% | 0.1969 | 5.00 | more torque, more broken taps |
| #9 | 83% | 0.1960 | 4.98 | more torque, more broken taps |

## Clearance hole

- Close fit: F (0.2570 in / 6.53 mm)
- Free fit: H (0.2660 in / 6.76 mm)

## Counterbore (socket head cap screw)

- AMES: diameter 7/16" (0.4375 in), depth 0.2500 in

Countersink (flat head socket screw): 82 degrees to 0.5310 in across for a flush head.

## Heat-set insert hole (3D prints)

- Generic (Albany County): 0.3230 in

## Thread repair (Helicoil / STI drill)

- AMES: 17/64" (0.2656 in / 6.75 mm)
- MISUMI: 17/64" (0.2656 in / 6.75 mm)

## FAQ

Q: What size drill do I use for a 1/4-20 tap?
A: Use a #7 (0.2010 in / 5.11 mm) drill. It leaves about 75% thread, the standard for general-purpose tapping.

Q: What metric drill can I use for a 1/4-20 tap?
A: 5.1 mm (0.2008 in / 5.10 mm) is the closest, giving about 76% thread. Anything from about 65% to 80% thread taps well in most materials.

Q: What size clearance hole for a 1/4-20 screw?
A: F (0.2570") for a close fit, H (0.2660") for a free fit.

Q: What hole size for a 1/4-20 heat-set insert?
A: 0.3230" in the brands listed here. Check your insert's own listing; the hole is sized for that insert's knurl.

Q: What is the metric equivalent of 1/4-20?
A: M6 is the closest size (6.00 mm vs 6.35 mm major diameter). The pitches differ, so the screws and tapped holes are not interchangeable.

## Sources

- Engineers Edge: machinist screw thread tap drill chart (ANSI): https://engineersedge.com/tap_drill_chart.htm
- Penn MEDesign: tap drill chart: https://medesign.seas.upenn.edu/index.php/Guides.TapDrillChart
- Fractory: imperial clearance hole sizes: https://fractory.com/?p=21622
- AMES: counterbore hole dimensions, inch (Machinery's Handbook): https://amesweb.info/Screws/Counterbore-Hole-Dimensions-Chart.aspx
- SC Fastening: flat head socket cap screw spec, ASME B18.3 (PDF): https://scfastening.com/wp-content/uploads/2017/06/flatsocketspec.pdf
- Eugene Fastener: flat head socket screw countersink guide: https://www.eugenefast.com/Home/Page/resources/flat-head-socket-screw-countersink-guide
- Albany County Fasteners: heat-set insert specs (PDF): https://www.albanycountyfasteners.com/media/64/3d/5a/1764619189/heat-set-insert-specs.pdf
- AMES: Helicoil tap sizes (STI): https://amesweb.info/screws/helicoil-tap-sizes.aspx
- MISUMI: Helicoil tap and drill size chart: https://blog.misumiusa.com/helicoil-tap-and-drill-size-chart/