Engineering · Reference
Thread spec library.
UNC, UNF, metric coarse, metric fine, NPT pipe, BSP parallel — with major diameter and recommended tap drill for each thread. Clean, table-first reference for engineers reviewing prints and machinists at the toolbox.
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01
UNC — Unified National Coarse
18 sizes
| Designation | Pitch | Major dia (in) | Tap drill |
|---|---|---|---|
| #1-64 | 64 | 0.0730 | #53 (0.0595) |
| #2-56 | 56 | 0.0860 | #50 (0.0700) |
| #4-40 | 40 | 0.1120 | #43 (0.0890) |
| #5-40 | 40 | 0.1250 | #38 (0.1015) |
| #6-32 | 32 | 0.1380 | #36 (0.1065) |
| #8-32 | 32 | 0.1640 | #29 (0.1360) |
| #10-24 | 24 | 0.1900 | #25 (0.1495) |
| #12-24 | 24 | 0.2160 | #16 (0.1770) |
| 1/4-20 | 20 | 0.2500 | #7 (0.2010) |
| 5/16-18 | 18 | 0.3125 | F (0.2570) |
| 3/8-16 | 16 | 0.3750 | 5/16 (0.3125) |
| 7/16-14 | 14 | 0.4375 | U (0.3680) |
| 1/2-13 | 13 | 0.5000 | 27/64 (0.4219) |
| 9/16-12 | 12 | 0.5625 | 31/64 (0.4844) |
| 5/8-11 | 11 | 0.6250 | 17/32 (0.5313) |
| 3/4-10 | 10 | 0.7500 | 21/32 (0.6563) |
| 7/8-9 | 9 | 0.8750 | 49/64 (0.7656) |
| 1-8 | 8 | 1.0000 | 7/8 (0.8750) |
02
UNF — Unified National Fine
19 sizes
| Designation | Pitch | Major dia (in) | Tap drill |
|---|---|---|---|
| #0-80 | 80 TPI | 0.0600 | 3/64 (0.0469) |
| #1-72 | 72 | 0.0730 | #53 (0.0595) |
| #2-64 | 64 | 0.0860 | #50 (0.0700) |
| #3-56 | 56 | 0.0990 | #45 (0.0820) |
| #4-48 | 48 | 0.1120 | #42 (0.0935) |
| #5-44 | 44 | 0.1250 | #37 (0.1040) |
| #6-40 | 40 | 0.1380 | #33 (0.1130) |
| #8-36 | 36 | 0.1640 | #29 (0.1360) |
| #10-32 | 32 | 0.1900 | #21 (0.1590) |
| 1/4-28 | 28 | 0.2500 | #3 (0.2130) |
| 5/16-24 | 24 | 0.3125 | I (0.2720) |
| 3/8-24 | 24 | 0.3750 | Q (0.3320) |
| 7/16-20 | 20 | 0.4375 | 25/64 (0.3906) |
| 1/2-20 | 20 | 0.5000 | 29/64 (0.4531) |
| 9/16-18 | 18 | 0.5625 | 33/64 (0.5156) |
| 5/8-18 | 18 | 0.6250 | 37/64 (0.5781) |
| 3/4-16 | 16 | 0.7500 | 11/16 (0.6875) |
| 7/8-14 | 14 | 0.8750 | 13/16 (0.8125) |
| 1-12 | 12 | 1.0000 | 59/64 (0.9219) |
03
Metric — Coarse (ISO standard)
14 sizes
| Designation | Pitch | Major dia (mm/in) | Tap drill |
|---|---|---|---|
| M1.6 × 0.35 | 0.35 mm | 1.60 (0.0630) | 1.25 mm (0.0492) |
| M2 × 0.4 | 0.40 | 2.00 (0.0787) | 1.6 mm (0.0630) |
| M2.5 × 0.45 | 0.45 | 2.50 (0.0984) | 2.05 mm (0.0807) |
| M3 × 0.5 | 0.50 | 3.00 (0.1181) | 2.5 mm (0.0984) |
| M4 × 0.7 | 0.70 | 4.00 (0.1575) | 3.3 mm (0.1299) |
| M5 × 0.8 | 0.80 | 5.00 (0.1969) | 4.2 mm (0.1654) |
| M6 × 1.0 | 1.00 | 6.00 (0.2362) | 5.0 mm (0.1969) |
| M8 × 1.25 | 1.25 | 8.00 (0.3150) | 6.8 mm (0.2677) |
| M10 × 1.5 | 1.50 | 10.00 (0.3937) | 8.5 mm (0.3346) |
| M12 × 1.75 | 1.75 | 12.00 (0.4724) | 10.2 mm (0.4016) |
| M14 × 2.0 | 2.00 | 14.00 (0.5512) | 12.0 mm (0.4724) |
| M16 × 2.0 | 2.00 | 16.00 (0.6299) | 14.0 mm (0.5512) |
| M20 × 2.5 | 2.50 | 20.00 (0.7874) | 17.5 mm (0.6890) |
| M24 × 3.0 | 3.00 | 24.00 (0.9449) | 21.0 mm (0.8268) |
04
Metric — Fine
10 sizes
| Designation | Pitch | Major dia (mm/in) | Tap drill |
|---|---|---|---|
| M3 × 0.35 | 0.35 mm | 3.00 (0.1181) | 2.65 mm (0.1043) |
| M4 × 0.5 | 0.50 | 4.00 (0.1575) | 3.5 mm (0.1378) |
| M5 × 0.5 | 0.50 | 5.00 (0.1969) | 4.5 mm (0.1772) |
| M6 × 0.75 | 0.75 | 6.00 (0.2362) | 5.25 mm (0.2067) |
| M8 × 1.0 | 1.00 | 8.00 (0.3150) | 7.0 mm (0.2756) |
| M10 × 1.25 | 1.25 | 10.00 (0.3937) | 8.8 mm (0.3465) |
| M12 × 1.5 | 1.50 | 12.00 (0.4724) | 10.5 mm (0.4134) |
| M14 × 1.5 | 1.50 | 14.00 (0.5512) | 12.5 mm (0.4921) |
| M16 × 1.5 | 1.50 | 16.00 (0.6299) | 14.5 mm (0.5709) |
| M20 × 1.5 | 1.50 | 20.00 (0.7874) | 18.5 mm (0.7283) |
05
NPT — National Pipe Thread (tapered)
7 sizes
| Designation | Pitch | Major dia (in) | Tap drill | Notes |
|---|---|---|---|---|
| 1/16-27 | 27 TPI | 0.3125 | C (0.2420) | Tapered 1°47′ |
| 1/8-27 | 27 | 0.4050 | R (0.3390) | Tapered 1°47′ |
| 1/4-18 | 18 | 0.5400 | 7/16 (0.4375) | Tapered 1°47′ |
| 3/8-18 | 18 | 0.6750 | 37/64 (0.5781) | Tapered 1°47′ |
| 1/2-14 | 14 | 0.8400 | 23/32 (0.7188) | Tapered 1°47′ |
| 3/4-14 | 14 | 1.0500 | 59/64 (0.9219) | Tapered 1°47′ |
| 1-11.5 | 11.5 | 1.3150 | 1-5/32 (1.1563) | Tapered 1°47′ |
06
BSP — British Standard Pipe (parallel)
6 sizes
| Designation | Pitch | Major dia | Tap drill |
|---|---|---|---|
| G1/8 (28 TPI) | 28 | 9.728 mm | 8.7 mm |
| G1/4 (19 TPI) | 19 | 13.157 mm | 11.7 mm |
| G3/8 (19 TPI) | 19 | 16.662 mm | 15.2 mm |
| G1/2 (14 TPI) | 14 | 20.955 mm | 18.6 mm |
| G3/4 (14 TPI) | 14 | 26.441 mm | 24.2 mm |
| G1 (11 TPI) | 11 | 33.249 mm | 30.5 mm |
07
Helicoil — STI tap-drill rule
Helicoil (and equivalent wire-thread inserts) require an oversize tap-drill compared to standard. The pre-drill diameter follows the STI (Screw Thread Insert) tap chart — typically the same drill that would be used for the next-size-up nominal thread.
- Inch: add ~0.012–0.020″ to standard tap-drill diameter, depending on thread size.
- Metric: add ~0.3–0.5 mm to standard tap-drill, depending on thread size.
- Always check the manufacturer’s STI chart for the specific insert series — Heli-Coil, Recoil, and Time-Sert have small differences.
- Tap with an STI tap, not a standard tap. STI taps are oversize to match the pre-drill.
When to specify which
Picking the right thread series.
- UNC for general assembly + soft material.
Coarse pitch tolerates rougher manufacturing and resists stripping in aluminum, cast iron, or plastic. Default for most non-critical structural fasteners.
- UNF for vibration + hard material.
Fine pitch resists loosening from vibration, gives finer torque control. Standard for aerospace structural fasteners and steel-on-steel.
- Metric coarse for international + ISO drawings.
Default international standard. M6, M8, M10 are the workhorses.
- Metric fine for precision + thin wall.
Better engagement on thin sections and finer pitch for fine-adjustment applications.
- NPT for tapered pipe sealing.
Tapered thread (1°47′ angle) seals by interference. Standard for North American hydraulic and pneumatic plumbing; usually paired with thread sealant.
- BSP for European pipe + fluid power.
G-series is parallel; R-series is tapered. Common on European hydraulic equipment. Not interchangeable with NPT.
For tap-drill recommendations and depth/torque calculations, see the tap-drill calculator and bolt-torque calculator.
Keep exploring
Related tools & references
Drill size for any thread at a chosen percent of thread engagement.
Number, letter, fractional, and metric drill decimal equivalents.
Through-hole sizes for bolt and screw pass-through fits.
Head recess dimensions for flat- and socket-head fasteners.
Minimum engagement length for full fastener strength.
Target torque from thread size, grade, and lubrication.
Technically reviewed by the True Precision Machining engineering team · Last reviewed June 2026
Standards referenced: ASME B1.1, ISO 261, ISO 262, ASME B1.13M
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