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1-8 bolt tightening torque

A 1-8 bolt in SAE Grade 5, dry, torques to about 644 lb-ft (873 N·m) at 75% of proof load.

1″ major dia · 8 TPI · At = 0.606 in² · clean, undamaged threads

Torque calculator T = K·D·F
Tightening torque
Tightening torque
T=K⋅D⋅FT = K \cdot D \cdot F
T torque · K nut factor (friction) · D nominal diameter · F preload
Preload (target clamp load)
F=u⋅Sp⋅AtF = u \cdot S_p \cdot A_t
u utilisation (0.75) · Sp proof strength · At tensile stress area

1-8 torque by grade and lubrication (lb-ft)

GradeDry / as-received steel
K=0.2		Zinc plated
K=0.22		Lubricated (light oil)
K=0.15		Molybdenum disulfide
K=0.1		Anti-seize compound
K=0.13		Waxed / PTFE
K=0.1
SAE Grade 2250275187125162125
SAE Grade 5644708483322418322
SAE Grade 8909999681454591454
18-8 Stainless303333227151197151

Proof strengths per SAE J429; tensile stress area per ASME B1.1. Values at 75 % of proof load.

Caution — match the column to your assembly. The same bolt can need half the torque lubricated versus dry, because K drops from ~0.20 to ~0.10. A dry-torque figure applied to a lubricated bolt over-tightens it and can snap it.

How the value is worked out

Torque isn't the goal — clamp load is. Tightening stretches the bolt into a controlled tension that squeezes the joint together. For a 1-8 in SAE Grade 8 at 75 % of proof load that target tension is about 54500 lbf; torque is just the indirect lever used to reach it, and the relationship runs through the nut factor K. The tensile stress area of the 1-8 thread is 0.606 in²; multiply it by the grade's proof strength and by 0.75 to get the preload F, then feed F into T = K·D·F for the table above.

Choosing the right friction condition

This is the input people get wrong most often. K bundles every friction loss in the joint — under the head and across the threads — and it moves the torque dramatically: dry, a 1-8 in SAE Grade 8 wants about 909 lb-ft; with light oil only 681 lb-ft; with MoS₂ roughly 454 lb-ft — for the same target tension. Always match the column to what is actually on the threads.

Grade matters as much as size

SAE Grade 2 tolerates roughly 250 lb-ft dry; SAE Grade 8 in the same diameter takes about 909 lb-ft because it safely holds far more tension. Torquing a lower grade to a higher grade's table is another classic way to fail a joint — check the head markings before picking a row. And remember: torque-wrench control scatters ±25 % in practice; for safety-critical joints use angle-control or direct tension measurement.

Tapping the mating hole? 1-8 tap drill size  ·  1 clearance hole

References

  1. SAE J429 — Mechanical and material requirements for externally threaded fasteners (proof strengths).
  2. ASME B1.1 — Unified inch screw threads: tensile stress area.
  3. Machinery’s Handbook — torque–preload relation and nut-factor scatter.

Last reviewed: 2026-07 · Calculation method & assumptions