Bolt Torque for 1-1/2-6 UNC

Torque Specifications for 1-1/2-6 UNC

The recommended tightening torque for a 1-1/2-6 UNC bolt depends on the bolt grade and whether the threads are lubricated. Below are the standard values for all common SAE grades:

Grade	Proof Load	Clamp Load	Torque (Dry)	Torque (Lubricated)
Grade 2	46,400 lbs	34,800 lbs	870 ft-lbs	435 ft-lbs
Grade 5	104,000 lbs	78,000 lbs	1950 ft-lbs	975 ft-lbs
Grade 8	168,600 lbs	126,450 lbs	3161 ft-lbs	1581 ft-lbs

Clamp loads based on 75% of minimum proof load per SAE J429. Torque calculated using T = K×D×P where K = 0.20 (dry) or 0.10 (lubricated).

Dry vs. Lubricated: Why It Matters

Lubrication has a dramatic effect on bolt torque. The same torque applied to a lubricated bolt produces roughly twice the clamp force compared to a dry bolt.

This means:

• Dry torque values assume clean, unlubricated threads (K ≈ 0.20). This is the default for most applications.
• Lubricated values apply when using anti-seize, oil, wax, or other thread lubricants (K ≈ 0.10).
• Critical: If you apply dry torque values to lubricated bolts, you will over-tighten them — potentially yielding or breaking the bolt.

Rule of thumb: Lubricated torque is approximately 50% of dry torque for the same bolt.

How Bolt Torque Is Calculated

Bolt torque specifications are derived from the standard torque-tension formula:

T = K × D × P

Where:

• T = Torque (inch-pounds, divided by 12 for ft-lbs)
• K = Torque coefficient (0.20 for dry, 0.10 for lubricated)
• D = Nominal bolt diameter (inches)
• P = Desired clamp load (lbs) — typically 75% of proof load

For a 1-1/2-6 UNC Grade 5 bolt (dry):

T = 0.20 × 1-1/2 × 78,000 lbs = 1950 ft-lbs

How to Identify Bolt Grade

SAE bolt grades are identified by the markings on the bolt head:

• Grade 2 — No markings (plain head). Low carbon steel, lowest strength.
• Grade 5 — Three radial lines (120° apart). Medium carbon steel, quenched and tempered. Most common structural grade.
• Grade 8 — Six radial lines (60° apart). Medium carbon alloy steel, quenched and tempered. Highest common grade.

If you can’t identify the grade, assume Grade 2 values for safety. Never assume a higher grade than you can verify.

Related Bolt Sizes

• 1-1/4-7 UNC Bolt Torque — Grade 5: 1120 ft-lbs (dry)
• 1-3/8-6 UNC Bolt Torque — Grade 5: 1469 ft-lbs (dry)