Bolt Torque Calculator (8.8 / 10.9 / 12.9)
Tightening torque for metric bolts M6-M30 by property class and lubrication — with preload force and the T = K·F·d formula.
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How Tightening Torque Is Calculated
The short-form formula is T = K · F · d: torque = nut factor × preload × nominal diameter. Preload is taken as 75% of proof load (proof stress × tensile stress area) — the standard assumption for reusable joints. Proof stress is 580-600 MPa for class 8.8, 830 for 10.9 and 970 for 12.9; stress areas come from the ISO metric coarse thread table.
The nut factor K dominates the result: ~0.20 dry, ~0.15 lightly oiled, ~0.12 with MoS₂ grease — which is why the same bolt gets a third less torque when lubricated. An M12 class 8.8 bolt, dry, works out to ≈ 88 N·m, matching published tables.
Critical joints (engines, structural steel, flanges) have their own specified torques and tightening sequences — the manufacturer's value always overrides a generic table.
Frequently Asked Questions
Why does lubrication change the torque so much?
Roughly 90% of applied torque fights friction (under the head and in the threads); only ~10% stretches the bolt. Cutting friction with oil or MoS₂ means the same preload needs much less torque — using the dry value on a lubricated bolt over-tensions and can snap it.
Can I reuse a torqued bolt?
Class 8.8-12.9 bolts tightened to 75% of proof stay elastic and are generally reusable if undamaged. Torque-to-yield bolts (common in engines, tightened by angle) stretch plastically and are single-use.
What is the difference between 8.8, 10.9 and 12.9?
The first number is tensile strength ÷ 100 (800/1000/1200 MPa); the second is the yield-to-tensile ratio (0.8/0.9). Higher class = more preload from the same size, but also more brittleness and hydrogen-embrittlement sensitivity in harsh environments.
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Bolt Torque Calculator (8.8 / 10.9 / 12.9)