What torque does a 5/8-18 UNF bolt take?
Answer
A 5/8-18 UNF in SAE Grade 5 takes 230.5 N·m — 170 lb·ft — dry, tightened to 75% of proof load. Oiled, it is far less.
How it is worked out
- Torque is only a proxy for tension
T = K × D × F - Tensile stress area of this bolt
165.13 mm² - Target 75% of proof load
72.58 kN of clamp force in SAE Grade 5 - Multiply out
0.20 × 0.0159 m × 72,583 N = 230.5 N·m
T = K · D · F — and 85–90% of that torque is spent on friction
Worth knowing
- Dry, as received — K 0.20
- SAE Grade 5 230.5 N·m (170 lb·ft) · SAE Grade 8 325.3 N·m (240 lb·ft) · SAE Grade 2 149.1 N·m (110 lb·ft)
- Lightly oiled — K 0.15
- SAE Grade 5 172.8 N·m · SAE Grade 8 244 N·m · SAE Grade 2 111.8 N·m
- Anti-seize or moly — K 0.12
- SAE Grade 5 138.3 N·m · SAE Grade 8 195.2 N·m · SAE Grade 2 89.5 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
- 82 kN in SAE Grade 5
- 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
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