4-40 tap drill size
Drill a #43 hole (0.0890" / 2.26 mm) for a 4-40 UNC tap. That leaves about 71% thread, the standard for general-purpose tapping.
4-40 tap drill: #43 (0.0890 in / 2.26 mm), 71% thread4-40 thread specs
| Series | Unified coarse (UNC) |
|---|---|
| Major diameter | 0.1120" (2.84 mm) |
| Pitch | 40 TPI (0.64 mm) |
| Basic minor diameter | 0.0849" (2.16 mm) |
| Common internal class | 2B |
Every drill that works for 4-40
Drills from all four sets that give 50–85% thread, largest first. Lower percentages tap easier; see how much thread you need.
| Drill | Thread | in | mm | Set | Good for |
|---|---|---|---|---|---|
| 2.4 mm | 54% | 0.0945 | 2.40 | Metric | deep holes only |
| 3/32" | 56% | 0.0938 | 2.38 | Fractional | deep holes only |
| #42 | 57% | 0.0935 | 2.37 | Number | deep holes only |
| 2.35 mm | 60% | 0.0925 | 2.35 | Metric | deep holes only |
| 2.3 mm | 66% | 0.0906 | 2.30 | Metric | hard or tough metals |
| #43standard | 71% | 0.0890 | 2.26 | Number | general use |
| 2.25 mm | 72% | 0.0886 | 2.25 | Metric | general use |
| 2.2 mm | 78% | 0.0866 | 2.20 | Metric | general use |
| #44 | 80% | 0.0860 | 2.18 | Number | more torque, more broken taps |
| 2.15 mm | 84% | 0.0846 | 2.15 | Metric | more torque, more broken taps |
4-40 clearance hole
The hole a 4-40 screw passes through. Close fit locates parts; free fit is forgiving.
| Fit | Drill | in | mm |
|---|---|---|---|
| Close fit | #32 | 0.1160 | 2.95 |
| Free fit | #30 | 0.1285 | 3.26 |
4-40 counterbore and countersink
For a socket head cap screw to sit flush, and for a flat head socket screw.
| Socket head counterbore | Diameter | Depth |
|---|---|---|
| AMES | 7/32" 0.2188" | 0.1120" |
Flat head socket screw: countersink at 82° (inch screws, ASME B18.3) to 0.2550" (6.48 mm) across for a flush head. Metric flat heads use 90°, so don't mix countersinks.
4-40 heat-set insert hole
Hole to model for a brass heat-set insert in a 3D print. Brands size their knurl differently, so use your brand’s number.
| Brand | Hole |
|---|---|
| Generic (Albany County) | 0.1290" |
4-40 holes in a 3D print
Printed holes usually come out undersize. Model the hole at the drill size plus what your printer loses, then tap or drill to size. Why printed holes shrink.
Print a test hole, measure it with calipers, enter the difference. 0 models the nominal size.
4-40 thread repair (Helicoil)
Stripped 4-40 thread? Drill out with the STI drill, tap with a 4-40 UNC STI tap, and fit a wire thread insert. The drill in your repair kit’s instructions wins over any chart.
| Chart | STI drill | in | mm |
|---|---|---|---|
| AMES | #31 | 0.1200 | 3.05 |
| MISUMI | #31 | 0.1200 | 3.05 |
Related sizes
Frequently asked questions
What size drill do I use for a 4-40 tap?
What metric drill can I use for a 4-40 tap?
What size clearance hole for a 4-40 screw?
What hole size for a 4-40 heat-set insert?
What is the metric equivalent of 4-40?
Sources
- Engineers Edge: machinist screw thread tap drill chart (ANSI) checked 2026-10-19
- Penn MEDesign: tap drill chart checked 2026-10-19
- Fractory: imperial clearance hole sizes checked 2026-10-19
- AMES: counterbore hole dimensions, inch (Machinery's Handbook) checked 2026-10-19
- SC Fastening: flat head socket cap screw spec, ASME B18.3 (PDF) checked 2026-10-19
- Eugene Fastener: flat head socket screw countersink guide checked 2026-10-19
- Albany County Fasteners: heat-set insert specs (PDF) checked 2026-10-19
- AMES: Helicoil tap sizes (STI) checked 2026-10-19
- MISUMI: Helicoil tap and drill size chart checked 2026-10-19
Spotted a wrong number? Email contact@clove3d.com and it gets fixed.
% thread = (major − drill) ÷ (1.299 × pitch) × 100. The standard drill comes from published charts and is checked against this formula on every build.