Bolt Torque Table
| Bolt size (in-TPI) | Grade 2 ft-lb | Grade 5 ft-lb | Grade 8 ft-lb |
|---|---|---|---|
| 1/4-20 | 5 | 8 | 12 |
| 5/16-18 | 11 | 17 | 25 |
| 3/8-16 | 20 | 31 | 44 |
| 7/16-14 | 32 | 49 | 70 |
| 1/2-13 | 49 | 75 | 106 |
| 9/16-12 | 70 | 109 | 154 |
| 5/8-11 | 97 | 150 | 212 |
| 3/4-10 | 172 | 266 | 376 |
| 7/8-9 | 167 | 430 | 606 |
| 1-8 | 250 | 644 | 909 |
A torque wrench does not measure tightness - it estimates clamp load, and the estimate is only as good as the numbers behind it. This table lists dry (unlubricated) torque specs for the three common SAE bolt grades across the ten sizes that cover automotive, structural and household work, from 1/4-20 to 1-8. Every value follows the standard shop formula T = K·F·d: a nut factor K of 0.2, a clamp load set at 75% of the bolt's proof strength, and the nominal diameter. Grade 2 proof strength is 55 ksi up to 3/4-inch and drops to 33 ksi above it, Grade 5 is 85 ksi, Grade 8 is 120 ksi - which is why the table's Grade 2 column is the only one that ever turns down.
The lubrication footnote is where real assemblies go wrong. The same 1/2-13 Grade 5 bolt that takes 75 ft-lb dry takes roughly half that with oil or anti-seize on the threads, because a lubricated nut factor near 0.1 converts every foot-pound of wrench input into roughly twice the bolt stretch. Torque the chart's dry number into a waxed or galvanized assembly and you are stretching the bolt toward yield; torque a lubed number into dry, rusty threads and the joint walks loose within a season. Pick the column that matches the threads you actually have.
How to use
- Identify the bolt grade from the head marking: no marks (Grade 2), three radial lines (Grade 5), six radial lines (Grade 8).
- Measure the diameter and count the threads per inch - 1/2-13 and 1/2-20 are different bolts with different specs.
- Read the dry torque from the table at that size and grade.
- Cut the value roughly in half for lubricated, waxed or galvanized-plated threads, and tighten in stages to seat the joint first.
Frequently asked questions
What torque should a 1/2-13 Grade 5 bolt get?
75 ft-lb dry, which works out to about 102 Nm. That number assumes clean, dry, unlubricated threads and a clamp load at 75% of the bolt's 85 ksi proof strength. With oil or anti-seize on the threads the same bolt reaches the same clamp at roughly 38-40 ft-lb - which is why reusing a dry-chart value on a lubed assembly is the classic way to twist a bolt head off.
Why does Grade 2 torque go DOWN at 7/8-inch?
Because big Grade 2 bolts are made of weaker steel. SAE J429 drops Grade 2 proof strength from 55 ksi to 33 ksi above 3/4-inch diameter, so the 7/8-9 row (166 ft-lb) sits below the 3/4-10 row (172 ft-lb) - the only non-monotonic column in the table. It surprises everyone eventually: the bigger Grade 2 bolt is less strong per square inch, and the spec protects you from trusting size alone.
How much does lubrication change torque values?
Typically 30-50% less torque for the same clamp load. The nut factor K falls from about 0.2 dry to 0.10-0.15 with oil, wax or anti-seize. Grade 8 charts publish the extreme case: the lubricated column is almost exactly half the dry column. If a joint was designed and tightened dry, dry is what the torque value means - and vice versa.
How do I convert ft-lb to Nm?
Multiply by 1.356. The table's 75 ft-lb is 101.7 Nm, 250 ft-lb is 339 Nm. Metric fasteners run the same physics with ISO property classes - an 8.8 behaves roughly like Grade 5 and a 10.9 between Grade 5 and Grade 8 - but published metric charts quote Nm directly, so convert only for a sanity check, not as a spec.
Can I tighten without a torque wrench?
You can, and everyone does for Grade 2 hardware, but know the failure modes. Undertorqued joints loosen from vibration; overtightened Grade 5 and 8 bolts yield quietly and lose their clamp within weeks. The wrench-free trick that actually helps: tighten to snug with a short wrench, then add a measured turn - about one-sixth of a turn is close to spec for many automotive sizes. Anything with a gasket, a safety recall or a warranty behind it gets a torque wrench.
Can I reuse a bolt after torquing to spec?
Grade 2 and Grade 5 usually yes, if the threads are clean and undamaged - reinstall at the same spec. Grade 8 stretched near its clamp ceiling, and any bolt specified torque-to-yield (common on modern cylinder heads and rod bolts), is a single-use part by design: it has permanently stretched to do its job. Reusing a stretched fastener gives you a bolt that reads torque but holds no clamp.