UNF Thread Specifications
UNF (Unified National Fine) threads per ASME B1.1. Fine threads have a larger minor diameter than coarse threads of the same nominal size, which means higher tensile strength and better performance in thin-walled parts.
| Size | TPI | Major Ø (in) | Pitch Ø (in) | Minor Ø (in) | Tensile Stress Area (in²) |
|---|---|---|---|---|---|
| #0-80 | 80 | 0.0600 | 0.0519 | 0.0438 | 0.00180 |
| #1-72 | 72 | 0.0730 | 0.0640 | 0.0550 | 0.00278 |
| #2-64 | 64 | 0.0860 | 0.0759 | 0.0657 | 0.00394 |
| #4-48 | 48 | 0.1120 | 0.0985 | 0.0849 | 0.00661 |
| #6-40 | 40 | 0.1380 | 0.1218 | 0.1055 | 0.01015 |
| #8-36 | 36 | 0.1640 | 0.1460 | 0.1279 | 0.01474 |
| #10-32 | 32 | 0.1900 | 0.1697 | 0.1494 | 0.0200 |
| 1/4-28 | 28 | 0.2500 | 0.2268 | 0.2036 | 0.0364 |
| 5/16-24 | 24 | 0.3125 | 0.2854 | 0.2584 | 0.0580 |
| 3/8-24 | 24 | 0.3750 | 0.3479 | 0.3209 | 0.0878 |
| 7/16-20 | 20 | 0.4375 | 0.4050 | 0.3725 | 0.1187 |
| 1/2-20 | 20 | 0.5000 | 0.4675 | 0.4350 | 0.1599 |
| 9/16-18 | 18 | 0.5625 | 0.5264 | 0.4903 | 0.203 |
| 5/8-18 | 18 | 0.6250 | 0.5889 | 0.5528 | 0.256 |
| 3/4-16 | 16 | 0.7500 | 0.7094 | 0.6688 | 0.373 |
| 7/8-14 | 14 | 0.8750 | 0.8286 | 0.7822 | 0.509 |
| 1-12 | 12 | 1.0000 | 0.9459 | 0.8917 | 0.663 |
| 1-1/4-12 | 12 | 1.2500 | 1.1959 | 1.1417 | 1.073 |
| 1-1/2-12 | 12 | 1.5000 | 1.4459 | 1.3917 | 1.581 |
UNF Advantages
- Higher tensile strength — compare: 1/2-20 UNF = 0.1599 in² vs 1/2-13 UNC = 0.1419 in² (13% more)
- Better in thin walls — more threads engage before punching through
- Finer adjustment — each turn moves less distance (1/TPI per turn)
- Vibration resistance — smaller helix angle resists self-loosening
UNF Disadvantages
- More sensitive to cross-threading — finer pitch means less room for error during assembly
- Strips easier in soft materials — fewer threads per inch of engagement length
- More susceptible to galling — especially in stainless steel
Related: UNF Tap Drill Sizes | UNC Thread Dimensions | Hex Bolt Grades & Properties