Bolt Torque for 3/8-16 UNC

Torque Specifications for 3/8-16 UNC

The recommended tightening torque for a 3/8-16 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	4,250 lbs	3,188 lbs	20 ft-lbs	10 ft-lbs
Grade 5	6,600 lbs	4,950 lbs	31 ft-lbs	15 ft-lbs
Grade 8	9,300 lbs	6,975 lbs	44 ft-lbs	22 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 3/8-16 UNC Grade 5 bolt (dry):

T = 0.20 × 3/8 × 4,950 lbs = 31 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

• 1/4-20 UNC Bolt Torque — Grade 5: 8 ft-lbs (dry)
• 5/16-18 UNC Bolt Torque — Grade 5: 17 ft-lbs (dry)
• 7/16-14 UNC Bolt Torque — Grade 5: 49 ft-lbs (dry)
• 1/2-13 UNC Bolt Torque — Grade 5: 75 ft-lbs (dry)