Proper bolt torque is critical for joint integrity. Under-torque leads to loosening and joint failure. Over-torque causes bolt stretch, thread stripping, or sudden fracture. This guide covers torque specifications for common fastener grades and the factors that affect them.
Understanding Bolt Grades
Bolt grades indicate tensile strength. The markings on the bolt head tell you exactly what you're working with:
SAE Grades (Imperial)
- Grade 2 (no markings): Low carbon steel, 74,000 PSI tensile. Hardware store bolts.
- Grade 5 (3 radial lines): Medium carbon, 120,000 PSI. Most common structural grade.
- Grade 8 (6 radial lines): Alloy steel, 150,000 PSI. High-strength applications.
Metric Classes
- Class 8.8: Equivalent to Grade 5 (800 MPa tensile, yield at 80%)
- Class 10.9: Equivalent to Grade 8 (1000 MPa tensile)
- Class 12.9: Ultra-high strength (1200 MPa tensile)
The Torque Formula
Bolt torque relates directly to the clamping force (preload) you're trying to achieve:
Torque = K × D × F
Where K is the friction factor (nut factor), D is the bolt diameter, and F is the desired preload. The challenge is that K varies significantly based on lubrication and surface conditions.
Calculate Exact Torque Values
Get precise torque specs for any bolt size and grade. Includes dry, lubricated, and anti-seize factors.
Standard Torque Values
These values assume clean, dry threads and are appropriate for most general applications:
| Size | Grade 5 (ft-lb) | Grade 8 (ft-lb) |
|---|---|---|
| 1/4-20 | 8 | 11 |
| 5/16-18 | 17 | 24 |
| 3/8-16 | 31 | 44 |
| 7/16-14 | 49 | 70 |
| 1/2-13 | 75 | 105 |
| 5/8-11 | 150 | 210 |
| 3/4-10 | 260 | 375 |
The Lubrication Factor
This is where most torque mistakes happen. Lubrication dramatically reduces friction, which means more of your torque goes into stretching the bolt (preload) rather than overcoming friction:
- Dry threads (K ≈ 0.20): Baseline torque values
- Light oil (K ≈ 0.18): Reduce torque by 10%
- Motor oil (K ≈ 0.15): Reduce torque by 25%
- Anti-seize (K ≈ 0.12): Reduce torque by 40%
- Moly paste (K ≈ 0.10): Reduce torque by 50%
Critical: If you apply full dry torque to a lubricated bolt, you can easily exceed yield strength and break the fastener.
Torque Sequence Matters
For flanges and multi-bolt patterns, proper sequence ensures even clamping:
- Start at 30% of final torque in a star/cross pattern
- Increase to 60%, maintaining the pattern
- Final pass at 100%
- Some specifications call for a final "check pass"
Going directly to full torque on the first bolt distorts the joint and leaves uneven clamping across the pattern.
When Standard Torque Doesn't Apply
Several situations require different approaches:
Torque-to-Yield (TTY) Bolts
Many modern engines and critical assemblies use TTY fasteners. These are torqued to a base value, then turned an additional specified angle (e.g., "25 ft-lb + 90°"). They cannot be reused.
Soft Materials
When bolting into aluminum, cast iron, or plastics, the limiting factor is the tapped thread, not the bolt. Reduce torque by 25-50% from steel-to-steel values.
High-Temperature Service
Elevated temperatures reduce material strength. Use temperature-rated fasteners and apply assembly torque at ambient conditions only.
Common Mistakes to Avoid
- Using impact wrenches for final torque: Use for run-down only, finish with a torque wrench
- Ignoring thread condition: Damaged or dirty threads change friction values
- Reusing stretch bolts: They're designed for one-time use
- Mixing bolt grades: Always replace with same or higher grade
- Skipping torque specs: "Tight" is not a torque value
Proper torque control is engineering, not guesswork. Understanding the relationship between torque, preload, and friction helps you make informed decisions when specifications aren't readily available or conditions vary from standard.
