1-1/2 inch bolt torque

A 1-1/2-6 bolt takes 869 ft-lb in Grade 2, 1949 in Grade 5 and 3161 in Grade 8, dry, at 75% of proof load; the 1-1/2-12 fine thread takes 978, 2194 and 3557. Oiled threads take a quarter less.

Sources checked

Torque at 75% of proof load.

Units
ThreadGradeDryK 0.20Zinc platedK 0.20OiledK 0.15WaxedK 0.10Clamp load
1-1/2-6UNC (coarse)Grade 28691,1798691,17965288443558934,774 lbf154.7 kN
Grade 51,9492,6431,9492,6431,4621,9829751,32277,978 lbf346.9 kN
Grade 83,1614,2863,1614,2862,3713,2151,5812,143126,450 lbf562.5 kN
1-1/2-12UNF (fine)Grade 29781,3269781,32673499548966339,130 lbf174.1 kN
Grade 52,1942,9742,1942,9741,6452,2311,0971,48787,746 lbf390.3 kN
Grade 83,5574,8233,5574,8232,6683,6171,7792,411142,290 lbf632.9 kN

Work out 1-1/2 inch torque

Change grade, threads or preload

Bolt standard

As received, nothing added. K 0.20 (Portland Bolt, Fastenal).

% of proof load

The published charts use 75%.

1-1/2-6 Grade 5, dry, plain steel

1,949ft-lb2,643N·m

0100020000100020003000ft-lbN·m
Clamp load
77,978 lbf (346.9 kN)
Proof load
103,970 lbf

The working

  1. Tensile stress area. A = 0.7854 × (D − 0.9743 ÷ n)² = 0.7854 × (1.5 − 0.9743 ÷ 6)² = 1.405 in²
  2. Proof load. Grade 5 proof stress is 74,000 psi, so A × stress = 103,970 lbf.
  3. Clamp load. 75% of proof load = 77,978 lbf.
  4. Torque. T = K × D × F = 0.2 × 1.5 in × 77,978 lbf = 23,393 in-lb ÷ 12 = 1,949 ft-lb (2,643 N·m).

Where the 1-1/2-6 numbers come from

The 1-1/2-6 UNC (coarse) thread has a tensile stress area of 1.405 in². A Grade 8 bolt’s proof stress is 120,000 psi, so its proof load is 168,600 lbf, and 75% of that is a clamp load of 126,450 lbf.

Torque = K × D × F = 0.20 × 1.5 in × 126,450 lbf = 37,935 in-lb, or 3,161 ft-lb (4,286 N·m) dry. With oil (K 0.15) it’s 2,371 ft-lb (3,215 N·m).

Other sizes

All sizes on one page: the SAE bolt torque chart.

Questions

What torque for a 1-1/2-6 Grade 8 bolt?

3,161 ft-lb (4,286 N·m) dry and 2,371 ft-lb (3,215 N·m) oiled, at 75% of proof load. That clamps the joint with 126,450 lbf (562.5 kN).

What torque for a 1-1/2-6 Grade 5 bolt?

1,949 ft-lb (2,643 N·m) dry, at 75% of proof load; a quarter less with oiled threads.

Coarse or fine thread?

Use whichever the bolt has. The fine 1-1/2-12 thread has a bigger stress area than the coarse one, so it takes slightly more torque for the same clamp. A thread gauge settles it.

Does this apply to wheel nuts or engine bolts?

No. Use the vehicle maker’s value for those: see lug nut torque by vehicle. These numbers are for general-purpose steel bolts where no spec exists.

Sources

  1. Portland Bolt: SAE J429: proof load, yield and tensile strength for Grades 2, 5 and 8 (checked October 14, 2026)
  2. Portland Bolt: Thread pitch chart: UNC and UNF threads per inch and tensile stress areas (checked October 14, 2026)
  3. Fastenal: Mechanical properties of inch fasteners, with UNC and UNF tensile stress areas (checked October 14, 2026)
  4. Engineering Toolbox: Threaded bolts: tensile stress area formulas for unified and ISO metric threads (checked October 14, 2026)
  5. Fastenal: Torque–tension relationship for A307 A, Grade 5, 8 and 9 coarse thread bolts (75% of proof load; K 0.15 and 0.20) (checked October 14, 2026)
  6. Portland Bolt: Bolt torque chart: T = KDP at 75% of proof load, with K for plain, waxed and galvanized bolts (checked October 14, 2026)

Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.