1-3/8 inch bolt torque

A 1-3/8-6 bolt takes 655 ft-lb in Grade 2, 1469 in Grade 5 and 2382 in Grade 8, dry, at 75% of proof load; the 1-3/8-12 fine thread takes 746, 1673 and 2712. 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
1-3/8-6UNC (coarse)Grade 265588865588849166632844428,586 lbf127.2 kN
Grade 51,4691,9921,4691,9921,1021,49473599664,103 lbf285.1 kN
Grade 82,3823,2302,3823,2301,7872,4221,1911,615103,950 lbf462.4 kN
1-3/8-12UNF (fine)Grade 27461,0117461,01155975837350632,546 lbf144.8 kN
Grade 51,6732,2681,6732,2681,2541,7018361,13472,983 lbf324.6 kN
Grade 82,7123,6772,7123,6772,0342,7581,3561,839118,350 lbf526.4 kN

Work out 1-3/8 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%.

1-3/8-6 Grade 5, dry, plain steel

1,469ft-lb1,992N·m

040080012001600010002000ft-lbN·m
Clamp load
64,103 lbf (285.1 kN)
Proof load
85,470 lbf

The working

  1. Tensile stress area. A = 0.7854 × (D − 0.9743 ÷ n)² = 0.7854 × (1.375 − 0.9743 ÷ 6)² = 1.155 in²
  2. Proof load. Grade 5 proof stress is 74,000 psi, so A × stress = 85,470 lbf.
  3. Clamp load. 75% of proof load = 64,103 lbf.
  4. Torque. T = K × D × F = 0.2 × 1.375 in × 64,103 lbf = 17,628 in-lb ÷ 12 = 1,469 ft-lb (1,992 N·m).

Where the 1-3/8-6 numbers come from

The 1-3/8-6 UNC (coarse) thread has a tensile stress area of 1.155 in². A Grade 8 bolt’s proof stress is 120,000 psi, so its proof load is 138,600 lbf, and 75% of that is a clamp load of 103,950 lbf.

Torque = K × D × F = 0.20 × 1.375 in × 103,950 lbf = 28,586 in-lb, or 2,382 ft-lb (3,230 N·m) dry. With oil (K 0.15) it’s 1,787 ft-lb (2,422 N·m).

Other sizes

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

Questions

What torque for a 1-3/8-6 Grade 8 bolt?

2,382 ft-lb (3,230 N·m) dry and 1,787 ft-lb (2,422 N·m) oiled, at 75% of proof load. That clamps the joint with 103,950 lbf (462.4 kN).

What torque for a 1-3/8-6 Grade 5 bolt?

1,469 ft-lb (1,992 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 1-3/8-12 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.