Every bolt size has a recommended torque range based on its diameter, thread pitch, and strength grade. Proper torque ensures consistent clamp force, prevents joint failure, and avoids thread damage. This chart provides baseline torque guidelines for common US bolt sizes from #1 to 2 inch across seven material grades, including both coarse (UNC) and fine (UNF) thread specifications. All values are in lb-ft for dry or slightly lubricated conditions.
Bolt Torque Lookup Tool
Select a bolt size to view recommended torque values across all material grades and thread types.
Quick Torque Lookup
Complete Bolt Torque Spec Chart
All torque values in lb-ft for dry, slightly lubricated bolts. Sizes #1–#10 are machine screw sizes; fractional sizes (1/4"–2") are standard bolt sizes. "Coarse" = UNC thread; "Fine" = UNF thread. A dash (—) indicates torque data not applicable for that size/grade combination.
| Размер | Grade 2 | Grade 5 | Grade 8 | 18-8 S/S | Bronze | Латунь | A2/A4 S/S | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Грубый | Fine | Грубый | Fine | Грубый | Fine | Грубый | Fine | Грубый | Fine | Грубый | Fine | Грубый | Fine | |
| Machine Screw Sizes (#1 – #10) | ||||||||||||||
| #1 | — | — | — | — | — | — | 0.3 | 0.2 | 0.2 | 0.3 | — | — | — | — |
| #2 | — | — | — | — | — | — | 0.6 | 0.5 | 0.4 | 0.6 | — | — | — | — |
| #3 | — | — | — | — | — | — | 1.1 | 0.9 | 0.8 | 1.0 | — | — | — | — |
| #4 | — | — | — | — | — | — | 5.2 | 4.8 | 4.3 | 5.0 | — | — | — | — |
| #5 | — | — | — | — | — | — | 9.5 | 8.9 | 7.9 | 9.2 | — | — | — | — |
| #6 | — | — | — | — | — | — | 14.5 | 13.5 | 12.0 | 14.0 | — | — | — | — |
| #8 | — | — | — | — | — | — | 19.8 | 18.4 | 16.2 | 19.0 | — | — | — | — |
| #10 | — | — | — | — | — | — | 22.8 | 31.7 | 21.2 | 29.3 | 18.6 | 25.9 | 22.0 | 30.5 |
| Standard Bolt Sizes (1/4" – 2") | ||||||||||||||
| 1/4" | 4 | 4.7 | 6.3 | 7.3 | 9 | 10 | 6.3 | 7.8 | 5.7 | 7.3 | 5.1 | 6.4 | 6.5 | 8.0 |
| 5/16" | 8 | 9 | 13 | 14 | 18 | 20 | 11 | 11.8 | 10.3 | 10.9 | 8.9 | 9.7 | 11.5 | 12.2 |
| 3/8" | 15 | 17 | 23 | 26 | 33 | 37 | 20 | 22 | 18 | 20 | 16 | 18 | 21 | 23 |
| 7/16" | 24 | 27 | 37 | 41 | 52 | 58 | 31 | 33 | 29 | 31 | 26 | 27 | 32 | 34 |
| 1/2" | 37 | 41 | 57 | 64 | 80 | 90 | 43 | 45 | 40 | 42 | 35 | 37 | 44 | 46 |
| 9/16" | 53 | 59 | 82 | 91 | 115 | 129 | 57 | 63 | 53 | 58 | 47 | 51 | 59 | 65 |
| 5/8" | 73 | 83 | 112 | 128 | 159 | 180 | 93 | 104 | 86 | 96 | 76 | 85 | 96 | 107 |
| 3/4" | 125 | 138 | 200 | 223 | 282 | 315 | 128 | 124 | 104 | 102 | 118 | 115 | 132 | 128 |
| 7/8" | 129 | 144 | 322 | 355 | 454 | 501 | 194 | 193 | 178 | 178 | 159 | 158 | 200 | 199 |
| 1" | 188 | 210 | 483 | 541 | 682 | 764 | 287 | 289 | 265 | 240 | 235 | 212 | 296 | 298 |
| 1-1/8" | 258 | 288 | 660 | 738 | 932 | 1043 | 394 | 396 | 363 | 329 | 322 | 290 | 406 | 408 |
| 1-1/4" | 340 | 380 | 870 | 974 | 1229 | 1376 | 520 | 523 | 479 | 434 | 425 | 383 | 536 | 539 |
| 1-3/8" | 435 | 486 | 1113 | 1246 | 1572 | 1760 | 665 | 669 | 613 | 555 | 543 | 490 | 686 | 690 |
| 1-1/2" | 543 | 607 | 1389 | 1555 | 1962 | 2196 | 830 | 835 | 765 | 693 | 678 | 612 | 856 | 861 |
| 1-3/4" | 840 | 939 | 2149 | 2406 | 3036 | 3399 | 1285 | 1292 | 1184 | 1073 | 1050 | 948 | 1325 | 1332 |
| 2" | 1210 | 1353 | 3095 | 3465 | 4371 | 4894 | 1850 | 1860 | 1705 | 1545 | 1512 | 1364 | 1907 | 1918 |
Bolt Grade Reference
Understanding bolt grades is essential for selecting the correct torque specification. Each grade has distinct mechanical properties that determine how much preload the fastener can safely handle.
Grade 2
Low/medium carbon steel. Tensile strength: 57,000 psi (up to 3/8"), 55,000 psi (over 3/8"). General-purpose, non-critical applications. Identified by no head markings.
Grade 5
Medium carbon steel, quenched & tempered. Tensile strength: 120,000 psi (up to 3/4"), 105,000 psi (over 3/4"). Most common for automotive and machinery. Identified by 3 radial lines on head.
Grade 8
Alloy steel, quenched & tempered. Tensile strength: 150,000 psi (up to 1-1/2"), 130,000 psi (over 1-1/2"). High-strength structural applications. Identified by 6 radial lines on head.
18-8 / 304 Stainless
Austenitic stainless steel (AISI 304). Tensile strength: 85,000 psi. Good corrosion resistance for indoor and mild outdoor environments. Prone to galling without lubrication. No grade lines on head.
Lubrication Torque Adjustment Factors
How Lubrication Changes Torque
The torque values in the chart above are for dry or slightly lubricated conditions (friction coefficient approximately 0.14). When you apply lubricant, friction decreases and more of the applied torque converts to clamp force. Using the dry torque value on a lubricated bolt can cause over-tightening, yield, or thread failure.
| Condition | Friction Coefficient | Torque Adjustment |
|---|---|---|
| Dry (as-charted) | approximately 0.14 | No adjustment — use chart values directly |
| Light oil / WD-40 | approximately 0.10 | Reduce torque by 15% |
| Motor oil / gear oil | approximately 0.08 | Reduce torque by 25% |
| Anti-seize compound | approximately 0.06 | Reduce torque by 30% |
| Molybdenum disulfide (MoS2) | approximately 0.05 | Reduce torque by 35% |
For critical joints, always measure preload directly with a load-inducing washer or strain gauge rather than relying on torque alone.
Tightening Best Practices
Multi-Bolt Joint Tightening Procedure
- Clean all threads — Remove dirt, rust, and old threadlocker. Damaged threads should be repaired with a tap or die before assembly.
- Lubricate consistently — Apply the same lubricant to all bolts in the joint. Mixed lubrication conditions cause uneven clamp load distribution.
- First pass: 30% of final torque — Tighten in a star or cross pattern to seat all components evenly and prevent warping.
- Second pass: 60% of final torque — Continue the star pattern. This further equalizes clamp load across the joint.
- Third pass: 100% of final torque — Complete the star pattern at full specification torque.
- Final circular pass — Go around the joint sequentially (not in star pattern) at 100% torque to verify uniformity. Mark each bolt with a paint pen after verification.
Часто задаваемые вопросы
Какой крутящий момент рекомендуется для болта 5 3/8 дюйма?
Рекомендуемый крутящий момент для болта 5, 3/8 дюйма класса составляет 23 фунта-фута для грубой резьбы (UNC) и 26 фунтов-фута для тонкой резьбы (UNF), в зависимости от сухих или слегка смазанных условий. Эти значения являются базовыми ориентирами. Всегда проверяйте спецификации производителя крепежа для критических применений, поскольку отделка поверхности, покрытие и смазка могут значительно повлиять на фактическое соотношение крутящего момента и напряжения.
В чем разница между класс прочности 2, класс прочности 5 и класс прочности 8 болты?
Болты 2 класса изготавливаются из низких или среднеуглеродистая сталь с минимальным предел прочности при растяжении 57 000 psi (для размеров до 3/8 дюйма). Болты 5 класса - среднеуглеродистая сталь, упрочненный и отпущенный, с минимальным предел прочности при растяжении 120 000 psi. 8 класс болты легированной стали, упрочненный и отпущенный, с минимальным предел прочности при растяжении 150 000 psi. По мере увеличения степени болт может выдерживать более высокую предварительную нагрузку, поэтому рекомендуемые значения крутящего момента соответственно увеличиваются.
Должен ли я использовать грубое или мелкая резьба крутящий момент значение?
Используйте значение крутящего момента, которое соответствует серии потоков вашего болта. крупная резьба (UNC) является стандартом для большинства приложений общего назначения. Тонкий поток (UNF) используется, когда требуется более высокое сопротивление вибрации или более тонкая регулировка. Если вы не уверены, проверьте шаг резьбы: грубые потоки имеют меньше витков на дюйм (например, 3/8-16 имеет 16 TPI), а тонкие потоки имеют больше (например, 3/8-24 имеет 24 TPI). Использование неправильного значения крутящего момента может привести к недостаточному или чрезмерному затягиванию.
How does lubrication affect bolt torque values?
Lubrication significantly reduces friction between the threads and under the bolt head, which means more of the applied torque converts to clamp force. The torque values in this chart are for dry or slightly lubricated bolts. If using oil, reduce the torque by approximately 15 percent. If using molybdenum disulfide (MoS2) or anti-seize compound, reduce torque by 20 to 30 percent. Over-torquing a lubricated bolt to the dry specification can cause yield or thread stripping.
What torque should I use for stainless steel bolts?
Stainless steel bolts (18-8 / 304 and A2/A4 / 316) have lower strength than Grade 5 steel bolts, so they require lower torque values. For example, a 3/8 inch 18-8 stainless bolt has a recommended torque of 20 lb-ft (coarse), compared to 23 lb-ft for Grade 5. Stainless steel is also prone to galling, especially when dry. Use a lubricant or anti-seize compound on stainless threads and reduce torque by an additional 10 to 15 percent to prevent galling during tightening.
Can I use a torque wrench to tighten small machine screws like #4 or #6?
Yes, but standard click-type torque wrenches are not accurate at very low torque values (below 5 lb-ft). For machine screws #0 through #10, use a precision inch-pound torque wrench or a torque screwdriver calibrated for the 1 to 50 in-lb range. The torque values in this chart are in lb-ft, so you will need to convert: multiply by 12 to get inch-pounds. For example, a #6 Grade 5 bolt at 14.5 lb-ft equals 174 inch-pounds.
What is the torque specification for a 1/2 inch Grade 8 bolt?
A 1/2 inch Grade 8 bolt has a recommended torque of 80 lb-ft for coarse thread (UNC, 1/2-13) and 90 lb-ft for fine thread (UNF, 1/2-20). These values assume dry or slightly lubricated conditions with a friction coefficient of approximately 0.14. For structural applications, always follow the applicable code requirements (such as RCSC specifications for A490 bolts, which is the structural equivalent of Grade 8).
How do I convert bolt torque from lb-ft to N-m?
To convert from lb-ft to Newton-meters (N-m), multiply the lb-ft value by 1.3558. For example, a 3/8 inch Grade 5 bolt at 23 lb-ft equals 23 x 1.3558 = 31.2 N-m. Common conversion factors: 1 lb-ft = 1.3558 N-m, 1 N-m = 0.7376 lb-ft, 1 lb-ft = 12 lb-in. Many torque wrenches are dual-calibrated in both units. For quick estimation, multiply lb-ft by 1.36 to get approximate N-m.
Why do fine thread bolts sometimes have different torque values than coarse?
Fine thread (UNF) and coarse thread (UNC) bolts of the same size and grade can have different torque specifications because fine threads have a larger tensile stress area (the minor diameter is larger) and a smaller thread pitch angle. The larger stress area allows slightly higher preload before yielding, while the smaller pitch angle changes the torque-tension relationship. For high-strength grades (5 and 8), fine thread torque values are typically slightly higher.
What tightening pattern should I use for multi-bolt flanges and joints?
For multi-bolt joints, always use a star or cross pattern to ensure even clamping and prevent distortion. The recommended procedure is: first pass at 30 percent of final torque, second pass at 60 percent, third pass at 100 percent, and a final circular pass at 100 percent to verify uniformity. Never tighten bolts sequentially in a circle, as this traps stress on one side and can warp the joint. For critical flanged connections, refer to ASME PCC-1 guidelines for bolted joint assembly.
Отказ от ответственности: The torque data provided in this chart is for general reference purposes only. Actual torque requirements may vary based on specific application conditions, surface finish, plating, lubrication, ambient temperature, and fastener manufacturer specifications. Always consult the original equipment manufacturer's recommendations, SAE J429, or applicable engineering standards before assembly. GUXIANG does not guarantee the accuracy or completeness of this data and assumes no liability for errors, omissions, or consequences arising from the use of this information.
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