M12x1.25 bolt tightening torque
A M12x1.25 bolt in Class 10.9, dry, torques to about 138 N·m (101 lb-ft) at 75% of proof load.
12 mm major dia · 1.25 mm pitch · At = 92.1 mm² · clean, undamaged threads
M12x1.25 torque by grade and lubrication (N·m)
| Grade | Dry / as-received steel K=0.2 | Zinc plated K=0.22 | Lubricated (light oil) K=0.15 | Molybdenum disulfide K=0.1 | Anti-seize compound K=0.13 | Waxed / PTFE K=0.1 |
|---|---|---|---|---|---|---|
| Class 8.8 | 96.1 | 106 | 72.1 | 48.1 | 62.5 | 48.1 |
| Class 10.9 | 138 | 151 | 103 | 68.8 | 89.4 | 68.8 |
| Class 12.9 | 161 | 177 | 121 | 80.4 | 104 | 80.4 |
| A2-70 Stainless | 74.6 | 82 | 55.9 | 37.3 | 48.5 | 37.3 |
Proof strengths per ISO 898-1 (ISO 3506-1 for A2-70); tensile stress area per ISO 898-1. Values at 75 % of proof load.
Caution — match the column to your assembly. The same bolt can need half the torque lubricated versus dry, because K drops from ~0.20 to ~0.10. A dry-torque figure applied to a lubricated bolt over-tightens it and can snap it.
How the value is worked out
Torque isn't the goal — clamp load is. Tightening stretches the bolt into a controlled tension that squeezes the joint together. For a M12x1.25 in Class 12.9 at 75 % of proof load that target tension is about 67 kN; torque is just the indirect lever used to reach it, and the relationship runs through the nut factor K. The tensile stress area of the M12x1.25 thread is 92.1 mm²; multiply it by the grade's proof strength and by 0.75 to get the preload F, then feed F into T = K·D·F for the table above.
Choosing the right friction condition
This is the input people get wrong most often. K bundles every friction loss in the joint — under the head and across the threads — and it moves the torque dramatically: dry, a M12x1.25 in Class 12.9 wants about 161 N·m; with light oil only 121 N·m; with MoS₂ roughly 80.4 N·m — for the same target tension. Always match the column to what is actually on the threads.
Grade matters as much as size
Class 8.8 tolerates roughly 96.1 N·m dry; Class 12.9 in the same diameter takes about 161 N·m because it safely holds far more tension. Torquing a lower grade to a higher grade's table is another classic way to fail a joint — check the head markings before picking a row. And remember: torque-wrench control scatters ±25 % in practice; for safety-critical joints use angle-control or direct tension measurement.
Tapping the mating hole? M12x1.25 tap drill size · M12 clearance hole
References
- ISO 898-1 — Mechanical properties of fasteners: property classes 8.8 / 10.9 / 12.9 (ISO 3506-1 for A2-70).
- ISO 724 / ISO 898-1 — metric thread dimensions and tensile stress area.
- Machinery’s Handbook — torque–preload relation and nut-factor scatter.
Last reviewed: 2026-07 · Calculation method & assumptions