What torque for an M10 8.8 bolt?

About 50 N·m dry, or about 38 N·m lightly oiled, for a preload near 25 kN.

What does 8.8 on a bolt head mean?

A tensile strength of about 800 N/mm² and a yield of about 80% of that (640 N/mm²).

Why is less torque used when the thread is oiled?

Oil lowers friction, so the same torque would stretch the bolt more. Using the dry figure on an oiled bolt can overload it.

Bolt torque calculator: metric bolts, grades 8.8 to 12.9

Bolt size (metric coarse)Property class (head marking)Thread condition

Tightening torque: 50.5 N·m
In kgf·m: 5.15 kgf·m
In lbf·ft: 37.2 lbf·ft
Clamping force (preload): 25.2 kN
Stress area: 58 mm²

Preload at 75% of proof load. A guide only: follow the machine maker's or the drawing's torque where given.

How it is worked out

Preload = 75% of the proof load = 0.75 × proof stress × stress area (ISO 898-1).
Torque (N·m) = nut factor K × preload (N) × nominal diameter (mm) ÷ 1000.
K is about 0.20 dry, 0.15 lightly oiled and 0.12 with anti-seize paste.

Questions

What torque for an M10 8.8 bolt?: About 50 N·m dry, or about 38 N·m lightly oiled, for a preload near 25 kN.
What does 8.8 on a bolt head mean?: A tensile strength of about 800 N/mm² and a yield of about 80% of that (640 N/mm²).
Why is less torque used when the thread is oiled?: Oil lowers friction, so the same torque would stretch the bolt more. Using the dry figure on an oiled bolt can overload it.

See also: Thread chart, Unit converter, and all calculators and references.

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Results use standard formulas and typical values. Use them for estimating and planning; check critical figures against the drawing, the material certificate or the maker's data.