Why Torque Matters
A bolt’s job is to clamp parts together with a specific force (preload). Torque is how we achieve that preload. Under-torque and the joint loosens. Over-torque and you strip threads, stretch the bolt, or crack the part.
Standard Torque Values — US Bolts
Dry torque values (no lubrication) for standard hex bolts. Reduce by 25–30% if lubricated.
SAE Grade 5 (3 radial lines on head)
| Bolt Size | TPI (UNC) | Torque (ft-lbs) | Torque (Nm) |
|---|---|---|---|
| 1/4″ | 20 | 8 | 11 |
| 5/16″ | 18 | 17 | 23 |
| 3/8″ | 16 | 31 | 42 |
| 7/16″ | 14 | 49 | 66 |
| 1/2″ | 13 | 75 | 102 |
| 9/16″ | 12 | 110 | 149 |
| 5/8″ | 11 | 150 | 203 |
| 3/4″ | 10 | 270 | 366 |
| 7/8″ | 9 | 395 | 536 |
| 1″ | 8 | 580 | 786 |
SAE Grade 8 (6 radial lines on head)
| Bolt Size | TPI (UNC) | Torque (ft-lbs) | Torque (Nm) |
|---|---|---|---|
| 1/4″ | 20 | 12 | 16 |
| 5/16″ | 18 | 24 | 33 |
| 3/8″ | 16 | 44 | 60 |
| 7/16″ | 14 | 70 | 95 |
| 1/2″ | 13 | 105 | 142 |
| 9/16″ | 12 | 155 | 210 |
| 5/8″ | 11 | 210 | 285 |
| 3/4″ | 10 | 380 | 515 |
| 7/8″ | 9 | 600 | 813 |
| 1″ | 8 | 900 | 1220 |
Metric Bolts
Class 8.8 (Equivalent to Grade 5)
| Bolt Size | Pitch | Torque (Nm) | Torque (ft-lbs) |
|---|---|---|---|
| M6 | 1.0 | 10 | 7 |
| M8 | 1.25 | 25 | 18 |
| M10 | 1.5 | 49 | 36 |
| M12 | 1.75 | 85 | 63 |
| M16 | 2.0 | 210 | 155 |
| M20 | 2.5 | 410 | 302 |
| M24 | 3.0 | 710 | 524 |
The Torque-Tension Relationship
Only about 10–15% of applied torque actually becomes bolt tension (clamping force). The rest is lost to friction — under the bolt head (~40%) and in the threads (~45%). This is why lubrication changes torque values dramatically.
Critical Rules
- Lubricated vs dry: A lubricated bolt at the same torque generates ~30% MORE clamping force than dry. If the torque spec is for dry, and you oil it, you’ll over-stress the bolt.
- Never reuse torque-to-yield bolts: TTY bolts (common in engines) are designed to stretch into the plastic zone. Reusing them risks failure.
- Star pattern: Multi-bolt flanges should be tightened in a star pattern, incrementally (50%, 75%, 100%), to ensure even clamping.
- Use a calibrated wrench: Hand-feel is unreliable, especially above 1/2″ bolts. A $30 torque wrench prevents thousands in warranty claims.
- Thread engagement: Minimum thread engagement = 1× bolt diameter in steel, 1.5× in aluminum, 2× in plastic. Less than that and the threads strip before the bolt yields.
Need torque-critical assemblies? We provide torque-controlled assembly with documented values for every fastener.