#4-40 bolt tightening torque

A #4-40 bolt in SAE Grade 5, dry, torques to about 0.718 lb-ft (0.973 N·m) at 75% of proof load.

0.112″ major dia · 40 TPI · A_t = 0.00603 in² · clean, undamaged threads

Torque calculator T = K·D·F

Grade / class

Lubrication

Preload, % of proof

Tightening torque

T = K \cdot D \cdot F

T torque · K nut factor (friction) · D nominal diameter · F preload

Preload (target clamp load)

F = u \cdot S_p \cdot A_t

u utilisation (0.75) · S_p proof strength · A_t tensile stress area

#4-40 torque by grade and lubrication (lb-ft)

Grade	Dry / 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 2	0.465	0.511	0.348	0.232	0.302	0.232
SAE Grade 5	0.718	0.79	0.538	0.359	0.467	0.359
SAE Grade 8	1.01	1.11	0.76	0.507	0.659	0.507
18-8 Stainless	0.338	0.372	0.253	0.169	0.22	0.169

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 #4-40 in SAE Grade 8 at 75 % of proof load that target tension is about 543 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 #4-40 thread is 0.00603 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 #4-40 in SAE Grade 8 wants about 1.01 lb-ft; with light oil only 0.76 lb-ft; with MoS₂ roughly 0.507 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 0.465 lb-ft dry; SAE Grade 8 in the same diameter takes about 1.01 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? #4-40 tap drill size · #4 clearance hole

References

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

Last reviewed: 2026-07 · Calculation method & assumptions