Torque values imperial fasteners
The tightening of unified thread (UTS) bolts is based on applying a tightening torque that generates sufficient axial clamping force to maintain the joint without exceeding the yield strength of the bolt. The values presented cover the UNC, UNF and UNEF series, and correspond to hex head bolts manufactured under SAE J429 standard in carbon and alloy steel.
Standard dimensions and pitches
Section titled “Standard dimensions and pitches”The nominal diameters and the number of threads per inch define each imperial thread size.
| UTS designation | Major diameter (in) | Major diameter (mm) | Series | TPI | Pitch (in) | Pitch (mm) |
|---|---|---|---|---|---|---|
| #0-80 UNF | 0.0600 | 1.524 | UNF | 80 | 0.012500 | 0.3175 |
| #1-64 UNC | 0.0730 | 1.8542 | UNC | 64 | 0.015625 | 0.3969 |
| #1-72 UNF | 0.0730 | 1.8542 | UNF | 72 | 0.013888 | 0.3528 |
| #2-56 UNC | 0.0860 | 2.1844 | UNC | 56 | 0.017857 | 0.4536 |
| #2-64 UNF | 0.0860 | 2.1844 | UNF | 64 | 0.015625 | 0.3969 |
| #3-48 UNC | 0.0990 | 2.5146 | UNC | 48 | 0.020833 | 0.5292 |
| #3-56 UNF | 0.0990 | 2.5146 | UNF | 56 | 0.017857 | 0.4536 |
| #4-40 UNC | 0.1120 | 2.8448 | UNC | 40 | 0.025000 | 0.6350 |
| #4-48 UNF | 0.1120 | 2.8448 | UNF | 48 | 0.020833 | 0.5292 |
| #5-40 UNC | 0.1250 | 3.1750 | UNC | 40 | 0.025000 | 0.6350 |
| #5-44 UNF | 0.1250 | 3.1750 | UNF | 44 | 0.022727 | 0.5773 |
| #6-32 UNC | 0.1380 | 3.5052 | UNC | 32 | 0.031250 | 0.7938 |
| #6-40 UNF | 0.1380 | 3.5052 | UNF | 40 | 0.025000 | 0.6350 |
| #8-32 UNC | 0.1640 | 4.1656 | UNC | 32 | 0.031250 | 0.7938 |
| #8-36 UNF | 0.1640 | 4.1656 | UNF | 36 | 0.027778 | 0.7056 |
| #10-24 UNC | 0.1900 | 4.8260 | UNC | 24 | 0.041667 | 1.0583 |
| #10-32 UNF | 0.1900 | 4.8260 | UNF | 32 | 0.031250 | 0.7938 |
| #12-24 UNC | 0.2160 | 5.4864 | UNC | 24 | 0.041667 | 1.0583 |
| #12-28 UNF | 0.2160 | 5.4864 | UNF | 28 | 0.035714 | 0.9071 |
| 1/4-20 UNC | 0.2500 | 6.3500 | UNC | 20 | 0.050000 | 1.2700 |
| 1/4-28 UNF | 0.2500 | 6.3500 | UNF | 28 | 0.035714 | 0.9071 |
| 1/4-32 UNEF | 0.2500 | 6.3500 | UNEF | 32 | 0.031250 | 0.7938 |
| 5/16-18 UNC | 0.3125 | 7.9375 | UNC | 18 | 0.055556 | 1.4111 |
| 5/16-24 UNF | 0.3125 | 7.9375 | UNF | 24 | 0.041667 | 1.0583 |
| 5/16-32 UNEF | 0.3125 | 7.9375 | UNEF | 32 | 0.031250 | 0.7938 |
| 3/8-16 UNC | 0.3750 | 9.5250 | UNC | 16 | 0.062500 | 1.5875 |
| 3/8-24 UNF | 0.3750 | 9.5250 | UNF | 24 | 0.041667 | 1.0583 |
| 3/8-32 UNEF | 0.3750 | 9.5250 | UNEF | 32 | 0.031250 | 0.7938 |
| 7/16-14 UNC | 0.4375 | 11.1125 | UNC | 14 | 0.071429 | 1.8143 |
| 7/16-20 UNF | 0.4375 | 11.1125 | UNF | 20 | 0.050000 | 1.2700 |
| 7/16-28 UNEF | 0.4375 | 11.1125 | UNEF | 28 | 0.035714 | 0.9071 |
| 1/2-13 UNC | 0.5000 | 12.7000 | UNC | 13 | 0.076923 | 1.9538 |
| 1/2-20 UNF | 0.5000 | 12.7000 | UNF | 20 | 0.050000 | 1.2700 |
| 9/16-12 UNC | 0.5625 | 14.2875 | UNC | 12 | 0.083333 | 2.1167 |
| 9/16-18 UNF | 0.5625 | 14.2875 | UNF | 18 | 0.055556 | 1.4111 |
| 5/8-11 UNC | 0.6250 | 15.8750 | UNC | 11 | 0.090909 | 2.3091 |
| 5/8-18 UNF | 0.6250 | 15.8750 | UNF | 18 | 0.055556 | 1.4111 |
| 3/4-10 UNC | 0.7500 | 19.0500 | UNC | 10 | 0.100000 | 2.5400 |
| 3/4-16 UNF | 0.7500 | 19.0500 | UNF | 16 | 0.062500 | 1.5875 |
| 7/8-9 UNC | 0.8750 | 22.2250 | UNC | 9 | 0.111111 | 2.8222 |
| 7/8-14 UNF | 0.8750 | 22.2250 | UNF | 14 | 0.071429 | 1.8143 |
| 1-8 UNC | 1.0000 | 25.4000 | UNC | 8 | 0.125000 | 3.1750 |
| 1-14 UNF | 1.0000 | 25.4000 | UNF | 14 | 0.071429 | 1.8143 |
Equivalences between standards
Section titled “Equivalences between standards”Although ISO metric and UTS thread profiles share the 60° angle, they are not directly interchangeable. The following table provides guidance on approximate substitution between imperial and metric sizes; thread pitches and design tolerances must always be verified.
| Imperial size | Approximate metric range | Nominal diameter (mm) |
|---|---|---|
| #6-32 | M3.5 – M4 | ≈ 3.5 |
| #8-32 | M4 | ≈ 4.2 |
| #10-24 | M5 | ≈ 4.8 |
| 1/4-20 | M6 | ≈ 6.35 |
| 5/16-18 | M8 | ≈ 7.94 |
| 3/8-16 | M10 | ≈ 9.53 |
| 7/16-14 | M11 – M12 | ≈ 11.1 |
| 1/2-13 | M12 – M14 | ≈ 12.7 |
| 9/16-12 | M14 | ≈ 14.3 |
| 5/8-11 | M16 | ≈ 15.9 |
| 3/4-10 | M18 – M20 | ≈ 19.1 |
| 7/8-9 | M22 | ≈ 22.2 |
| 1-8 | M24 – M27 | ≈ 25.4 |
Mechanical properties and allowable loads
Section titled “Mechanical properties and allowable loads”Mechanical properties are governed by SAE J429 for bolts from ¼ to 1 inch. The design preload is usually taken as 75% of the proof load or 70% of the ultimate tensile strength.
| SAE Grade | Proof load | Tensile strength |
|---|---|---|
| 2 | 55 000 psi / 379 MPa | 74 000 psi / 510 MPa |
| 5 | 85 000 psi / 586 MPa | 120 000 psi / 827 MPa |
| 8 | 120 000 psi / 827 MPa | 150 000 psi / 1034 MPa |
The tensile stress areas and recommended maximum tightening loads (75% of proof) are detailed below.
| Size | Series | Tensile area (in² / mm²) | Max preload Grade 2 (lb / N) | Max preload Grade 5 (lb / N) | Max preload Grade 8 (lb / N) |
|---|---|---|---|---|---|
| 1/4 | UNC | 0.0318 / 20.5 | 1 310 / 5 830 | 2 025 / 9 010 | 2 860 / 12 720 |
| 1/4 | UNF | 0.0364 / 23.5 | 1 500 / 6 670 | 2 320 / 10 320 | 3 280 / 14 580 |
| 5/16 | UNC | 0.0524 / 33.8 | 2 160 / 9 610 | 3 340 / 14 860 | 4 720 / 20 990 |
| 5/16 | UNF | 0.0580 / 37.4 | 2 390 / 10 640 | 3 700 / 16 450 | 5 220 / 23 230 |
| 3/8 | UNC | 0.0775 / 50.0 | 3 200 / 14 230 | 4 940 / 21 970 | 6 980 / 31 030 |
| 3/8 | UNF | 0.0878 / 56.6 | 3 620 / 16 100 | 5 600 / 24 900 | 7 900 / 35 150 |
| 7/16 | UNC | 0.1063 / 68.6 | 4 380 / 19 480 | 6 780 / 30 150 | 9 570 / 42 560 |
| 7/16 | UNF | 0.1187 / 76.6 | 4 900 / 21 810 | 7 560 / 33 640 | 10 680 / 47 510 |
| 1/2 | UNC | 0.1419 / 91.5 | 5 850 / 26 030 | 9 050 / 40 260 | 12 770 / 56 800 |
| 1/2 | UNF | 0.1599 / 103.1 | 6 600 / 29 370 | 10 190 / 45 340 | 14 390 / 64 010 |
| 9/16 | UNC | 0.1820 / 117.4 | 7 510 / 33 410 | 11 600 / 51 610 | 16 380 / 72 850 |
| 9/16 | UNF | 0.2030 / 130.9 | 8 370 / 37 240 | 12 940 / 57 560 | 18 270 / 81 250 |
| 5/8 | UNC | 0.2260 / 145.8 | 9 320 / 41 470 | 14 410 / 64 110 | 20 340 / 90 460 |
| 5/8 | UNF | 0.2560 / 165.1 | 10 560 / 46 970 | 16 320 / 72 610 | 23 040 / 102 480 |
| 3/4 | UNC | 0.3340 / 215.4 | 13 780 / 61 310 | 21 290 / 94 710 | 30 060 / 133 720 |
| 3/4 | UNF | 0.3730 / 240.6 | 15 390 / 68 470 | 23 780 / 105 780 | 33 570 / 149 340 |
| 7/8 | UNC | 0.4620 / 298.0 | 19 060 / 84 780 | 29 450 / 131 010 | 41 580 / 184 950 |
| 7/8 | UNF | 0.5090 / 328.3 | 20 990 / 93 410 | 32 450 / 144 350 | 45 810 / 203 760 |
| 1 | UNC | 0.6060 / 390.9 | 24 990 / 111 180 | 38 630 / 171 830 | 54 540 / 242 580 |
| 1 | UNF | 0.6800 / 438.6 | 28 040 / 124 760 | 43 350 / 192 830 | 61 200 / 272 210 |
Torque values table
Section titled “Torque values table”The torques shown correspond to a friction coefficient K = 0.20 (dry surfaces, black finish). For lubricated conditions, refer to the correction factors section.
| Size | Series | Dry torque Grade 2 (lb·ft / N·m) | Dry torque Grade 5 (lb·ft / N·m) | Dry torque Grade 8 (lb·ft / N·m) | Lubricated torque Grade 2 (lb·ft / N·m) | Lubricated torque Grade 5 (lb·ft / N·m) | Lubricated torque Grade 8 (lb·ft / N·m) |
|---|---|---|---|---|---|---|---|
| 1/4 | UNC | 4.0 / 5.4 | 6.2 / 8.4 | 8.8 / 11.9 | 2.4 / 3.3 | 3.7 / 5.0 | 5.3 / 7.2 |
| 1/4 | UNF | 4.7 / 6.4 | 7.3 / 9.9 | 10.3 / 14.0 | 2.8 / 3.8 | 4.4 / 6.0 | 6.2 / 8.4 |
| 5/16 | UNC | 8.4 / 11.4 | 13.0 / 17.6 | 18.4 / 24.9 | 5.0 / 6.8 | 7.8 / 10.6 | 11.0 / 14.9 |
| 5/16 | UNF | 9.7 / 13.2 | 15.0 / 20.3 | 21.2 / 28.7 | 5.8 / 7.9 | 9.0 / 12.2 | 12.7 / 17.2 |
| 3/8 | UNC | 15.0 / 20.3 | 23.2 / 31.4 | 32.7 / 44.3 | 9.0 / 12.2 | 13.9 / 18.8 | 19.6 / 26.6 |
| 3/8 | UNF | 17.8 / 24.1 | 27.5 / 37.3 | 38.8 / 52.6 | 10.7 / 14.5 | 16.5 / 22.4 | 23.3 / 31.6 |
| 7/16 | UNC | 24.0 / 32.5 | 37.1 / 50.3 | 52.4 / 71.0 | 14.4 / 19.5 | 22.3 / 30.2 | 31.4 / 42.6 |
| 7/16 | UNF | 28.0 / 38.0 | 43.2 / 58.6 | 61.0 / 82.7 | 16.8 / 22.8 | 25.9 / 35.2 | 36.6 / 49.6 |
| 1/2 | UNC | 36.6 / 49.6 | 56.5 / 76.6 | 79.8 / 108.2 | 22.0 / 29.8 | 33.9 / 46.0 | 47.9 / 64.9 |
| 1/2 | UNF | 43.2 / 58.6 | 66.8 / 90.6 | 94.3 / 127.8 | 25.9 / 35.2 | 40.1 / 54.4 | 56.6 / 76.7 |
| 9/16 | UNC | 52.8 / 71.6 | 81.7 / 110.7 | 115.3 / 156.3 | 31.7 / 43.0 | 49.0 / 66.4 | 69.2 / 93.8 |
| 9/16 | UNF | 61.8 / 83.7 | 95.5 / 129.5 | 134.8 / 182.8 | 37.1 / 50.2 | 57.3 / 77.7 | 80.9 / 109.7 |
| 5/8 | UNC | 72.8 / 98.7 | 112.6 / 152.7 | 158.9 / 215.5 | 43.7 / 59.2 | 67.6 / 91.6 | 95.3 / 129.3 |
| 5/8 | UNF | 86.6 / 117.5 | 133.8 / 181.5 | 188.9 / 256.2 | 52.0 / 70.5 | 80.3 / 108.9 | 113.3 / 153.7 |
| 3/4 | UNC | 129.3 / 175.3 | 199.8 / 270.9 | 282.1 / 382.5 | 77.6 / 105.2 | 119.9 / 162.6 | 169.3 / 229.5 |
| 3/4 | UNF | 151.5 / 205.4 | 234.1 / 317.4 | 330.4 / 448.0 | 90.9 / 123.2 | 140.5 / 190.5 | 198.2 / 268.8 |
| 7/8 | UNC | 208.6 / 282.8 | 322.3 / 437.0 | 455.0 / 617.0 | 125.1 / 169.7 | 193.4 / 262.2 | 273.0 / 370.2 |
| 7/8 | UNF | 241.3 / 327.2 | 372.8 / 505.5 | 526.2 / 713.5 | 144.8 / 196.3 | 223.7 / 303.3 | 315.7 / 428.1 |
| 1 | UNC | 312.3 / 423.5 | 482.5 / 654.2 | 681.1 / 923.5 | 187.4 / 254.1 | 289.5 / 392.5 | 408.7 / 554.1 |
| 1 | UNF | 368.0 / 498.9 | 568.6 / 771.0 | 802.6 / 1088.2 | 220.8 / 299.4 | 341.2 / 462.6 | 481.6 / 653.0 |
Correction factors for lubrication
Section titled “Correction factors for lubrication”Lubricant reduces friction between threads and bearing surface, so the tightening torque must be reduced to achieve the same preload. For lubricated bolts, it is recommended to multiply the dry torque by the following factors:
| Thread condition | Typical friction coefficient K | Correction factor on dry torque |
|---|---|---|
| Dry, black finish | 0.20 | 1.00 |
| Zinc plated | 0.20 | 1.00 |
| Slightly lubricated | 0.18 | 0.90 |
| Cadmium plated | 0.16 | 0.80 |
| Lubricated with SAE 30 oil | ≈ 0.12 | 0.60 |
Applying dry tightening torque to a lubricated bolt can generate excessive stress and cause immediate failure.
Tap drill sizes
Section titled “Tap drill sizes”The recommended drill diameters for taps ensure 75% thread height, suitable for most general applications.
| Size | Series | Drill inch (in) | Drill mm | Common designation |
|---|---|---|---|---|
| #0-80 UNF | UNF | 0.0469 (3/64) | 1.19 | 3/64 in |
| #1-64 UNC | UNC | 0.0595 | 1.51 | #53 |
| #1-72 UNF | UNF | 0.0595 | 1.51 | #53 |
| #2-56 UNC | UNC | 0.0700 | 1.78 | #50 |
| #2-64 UNF | UNF | 0.0700 | 1.78 | #50 |
| #3-48 UNC | UNC | 0.0785 | 1.99 | #47 |
| #3-56 UNF | UNF | 0.0820 | 2.08 | #45 |
| #4-40 UNC | UNC | 0.0890 | 2.26 | #43 |
| #4-48 UNF | UNF | 0.0935 | 2.37 | #42 |
| #5-40 UNC | UNC | 0.1015 | 2.58 | #38 |
| #5-44 UNF | UNF | 0.1040 | 2.64 | #37 |
| #6-32 UNC | UNC | 0.1065 | 2.71 | #36 |
| #6-40 UNF | UNF | 0.1130 | 2.87 | #33 |
| #8-32 UNC | UNC | 0.1360 | 3.45 | #29 |
| #8-36 UNF | UNF | 0.1360 | 3.45 | #29 |
| #10-24 UNC | UNC | 0.1495 | 3.80 | #25 |
| #10-32 UNF | UNF | 0.1590 | 4.04 | #21 |
| 1/4-20 UNC | UNC | 0.2010 | 5.11 | #7 |
| 1/4-28 UNF | UNF | 0.2130 | 5.41 | #3 |
| 5/16-18 UNC | UNC | 0.2570 (F) | 6.53 | F |
| 5/16-24 UNF | UNF | 0.2720 (I) | 6.91 | I |
| 3/8-16 UNC | UNC | 0.3125 (5/16) | 7.94 | 5/16 in |
| 3/8-24 UNF | UNF | 0.3320 (Q) | 8.43 | Q |
| 7/16-14 UNC | UNC | 0.3680 (U) | 9.35 | U |
| 7/16-20 UNF | UNF | 0.3906 (25/64) | 9.92 | 25/64 in |
| 1/2-13 UNC | UNC | 0.4219 (27/64) | 10.72 | 27/64 in |
| 1/2-20 UNF | UNF | 0.4531 (29/64) | 11.51 | 29/64 in |
| 9/16-12 UNC | UNC | 0.4844 (31/64) | 12.30 | 31/64 in |
| 9/16-18 UNF | UNF | 0.5156 (33/64) | 13.10 | 33/64 in |
| 5/8-11 UNC | UNC | 0.5312 (17/32) | 13.49 | 17/32 in |
| 5/8-18 UNF | UNF | 0.5781 (37/64) | 14.68 | 37/64 in |
| 3/4-10 UNC | UNC | 0.6562 (21/32) | 16.67 | 21/32 in |
| 3/4-16 UNF | UNF | 0.6875 (11/16) | 17.46 | 11/16 in |
| 7/8-9 UNC | UNC | 0.7656 (49/64) | 19.45 | 49/64 in |
| 7/8-14 UNF | UNF | 0.8125 (13/16) | 20.64 | 13/16 in |
| 1-8 UNC | UNC | 0.8750 (7/8) | 22.23 | 7/8 in |
| 1-14 UNF | UNF | 0.9375 (15/16) | 23.81 | 15/16 in |
Formulas and calculation principles
Section titled “Formulas and calculation principles”The fundamental relationship between tightening torque and clamping load is given by:
T = K × F × d × (1 – l / 100)
where:
- T = tightening torque (lb·ft, N·m)
- K = friction constant (typically 0.20 for dry steel bolts)
- F = axial clamping force (lb, N)
- d = nominal bolt diameter (ft, m)
- l = percentage reduction due to lubrication (%)
For a dry 3/4“-10 UNC bolt with target preload of 20 000 lb:
Tdry = 0.20 × 20 000 lb × 0.75 in × (1/12) ft/in = 250 lb·ft / 339 N·m.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the recommended dry tightening torque for a 1/2-13 UNC grade 5 bolt?
Section titled “What is the recommended dry tightening torque for a 1/2-13 UNC grade 5 bolt?”The recommended dry tightening torque is 57 lb·ft / 77 N·m, based on a preload equivalent to 75% of the proof load.
What dry torque does a 3/8-24 UNF grade 8 bolt require?
Section titled “What dry torque does a 3/8-24 UNF grade 8 bolt require?”For a dry 3/8-24 UNF grade 8, approximately 39 lb·ft / 53 N·m is needed.
How does lubrication with SAE 30 oil affect the torque of a 5/8-11 UNC grade 5 bolt?
Section titled “How does lubrication with SAE 30 oil affect the torque of a 5/8-11 UNC grade 5 bolt?”With SAE 30 lubrication, the torque is reduced to 68 lb·ft / 92 N·m, about 60% of the dry value of 113 lb·ft / 153 N·m.
Is there a difference in torque between fine (UNF) and coarse (UNC) threads for the same diameter and grade?
Section titled “Is there a difference in torque between fine (UNF) and coarse (UNC) threads for the same diameter and grade?”Yes, UNF bolts typically require 10% to 20% higher torque than UNC because they have a larger tensile stress area. For example, a 1/2-20 UNF grade 5 requires 67 lb·ft / 91 N·m, compared to 57 lb·ft / 77 N·m for a 1/2-13 UNC.
What drill size is used for tapping a 3/8-16 UNC hole?
Section titled “What drill size is used for tapping a 3/8-16 UNC hole?”The recommended drill is 5/16 in / 7,94 mm, which corresponds to the standard inch drill 5/16.
What is the maximum dry torque for a 1-8 UNC grade 8 bolt?
Section titled “What is the maximum dry torque for a 1-8 UNC grade 8 bolt?”The maximum dry torque is 681 lb·ft / 924 N·m; exceeding this value may exceed the yield strength and permanently damage the bolt.