What torque does a 1/2-13 UNC bolt take?
Answer
A 1/2-13 UNC in SAE Grade 5 takes 102.2 N·m — 75.4 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
91.55 mm² - Target 75% of proof load
40.24 kN of clamp force in SAE Grade 5 - Multiply out
0.20 × 0.0127 m × 40,239 N = 102.2 N·m
T = K · D · F — and 85–90% of that torque is spent on friction
Worth knowing
- Dry, as received — K 0.20
- SAE Grade 5 102.2 N·m (75.4 lb·ft) · SAE Grade 8 144.3 N·m (106.4 lb·ft) · SAE Grade 2 66.1 N·m (48.8 lb·ft)
- Lightly oiled — K 0.15
- SAE Grade 5 76.7 N·m · SAE Grade 8 108.2 N·m · SAE Grade 2 49.6 N·m
- Anti-seize or moly — K 0.12
- SAE Grade 5 61.3 N·m · SAE Grade 8 86.6 N·m · SAE Grade 2 39.7 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
- 45.4 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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