What torque does an M5 × 0.8 bolt take?

How it is worked out

1. Torque is only a proxy for tensionT = K × D × F
2. Tensile stress area of this bolt14.18 mm²
3. Target 75% of proof load6.17 kN of clamp force in Class 8.8
4. Multiply out0.20 × 0.005 m × 6,169 N = 6.2 N·m

T = K · D · F — and 85–90% of that torque is spent on friction

Worth knowing

Dry, as received — K 0.20

Class 8.8 6.2 N·m (4.6 lb·ft) · Class 10.9 8.8 N·m (6.5 lb·ft) · Class 12.9 10.3 N·m (7.6 lb·ft) · A2-70 stainless 4.8 N·m (3.5 lb·ft)

Lightly oiled — K 0.15

Class 8.8 4.6 N·m · Class 10.9 6.6 N·m · Class 12.9 7.7 N·m · A2-70 stainless 3.6 N·m

Anti-seize or moly — K 0.12

Class 8.8 3.7 N·m · Class 10.9 5.3 N·m · Class 12.9 6.2 N·m · A2-70 stainless 2.9 N·m

Why lubrication changes it so violently

Almost all the torque goes into friction rather than tension. Halving the friction roughly doubles the preload for the same torque — which is how a bolt gets snapped with a correctly set wrench and a dry-torque chart.

Single shear capacity

6.8 kN in Class 8.8

Can it be reused

At 75% of proof, yes — the bolt stayed elastic. Torque-to-yield fasteners have already stretched permanently and cannot.

The thread it goes into

This sizes the bolt, not the hole. A steel bolt in aluminium strips the aluminium first unless engagement is one and a half diameters or more.

Every grade at this size

Nearby sizes

Do it with your own numbers

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