Bolt Torque for M8 x 1.25

Torque Specifications for M8 x 1.25

The recommended tightening torque for an M8 x 1.25 bolt varies by property class and lubrication condition. Below are the standard values for the three most common metric classes:

Values per ISO 898-1. Dry K ≈ 0.20, lubricated K ≈ 0.10. Clamp load at 75% of proof load.

Understanding Metric Bolt Classes

Metric bolt property classes are marked with two numbers separated by a dot (e.g., “8.8”) stamped on the bolt head:

• Class 8.8 — Structural grade. Proof load ~580-600 MPa. The most common general-purpose high-strength metric bolt.
• Class 10.9 — Automotive/heavy-duty grade. Proof load ~830 MPa. Common in engine, suspension, and structural applications.
• Class 12.9 — Maximum common grade. Proof load ~970 MPa. Used in critical high-load applications.

The first number × 100 = approximate tensile strength in MPa. The second number × 10 = yield-to-tensile ratio percentage. So “8.8” means ~800 MPa tensile, ~640 MPa yield (80% of 800).

Dry vs. Lubricated Torque

Thread lubrication dramatically reduces the friction in the bolt threads and under the bolt head. For the same applied torque, a lubricated bolt develops roughly twice the clamping force compared to dry.

This means you must reduce torque by approximately 50% when using anti-seize, oil, or any thread lubricant. Using dry torque values on lubricated bolts will over-stress the fastener.

Nm to ft-lbs Conversion

The conversion factor between Newton-meters and foot-pounds is:

1 Nm = 0.7376 ft-lbs   |   1 ft-lb = 1.3558 Nm

For the M8 x 1.25 Class 8.8 bolt: 25.5 Nm × 0.7376 = 18.8 ft-lbs

Related Bolt Sizes

• M6 x 1 Bolt Torque — Class 8.8: 10.4 Nm (dry)
• M7 x 1 Bolt Torque — Class 8.8: 17.6 Nm (dry)
• M10 x 1.5 Bolt Torque — Class 8.8: 50.4 Nm (dry)
• M12 x 1.75 Bolt Torque — Class 8.8: 88.0 Nm (dry)