Grade 5 and Grade 8 are the most common high-strength fastener grades in North America. Understanding their differences helps you select the right bolt for your application without over-specifying (wasting money) or under-specifying (risking failure).
At a Glance
| Property | Grade 5 | Grade 8 |
|---|---|---|
| Material | Medium carbon steel | Alloy steel |
| Tensile Strength | 120,000 psi | 150,000 psi |
| Yield Strength | 92,000 psi | 130,000 psi |
| Head Markings | 3 radial lines | 6 radial lines |
| Cost | Lower | ~30-50% more |
Head Marking Identification
Count the radial lines on the bolt head:
- No markings: Grade 2 (low carbon, hardware store bolts)
- 3 lines: Grade 5 (most common structural grade)
- 6 lines: Grade 8 (high-strength applications)
Metric bolts use numbers (8.8, 10.9, 12.9) stamped on the head instead.
Get Torque Specs for Your Bolt Grade
Calculate proper tightening torque for Grade 5, Grade 8, and metric fasteners.
Strength Comparison
Grade 8 is about 25% stronger than Grade 5. In practical terms, for the same bolt size:
- Grade 8 can handle ~25% more load before yielding
- Grade 8 requires ~25% higher torque for equivalent preload
- Both can be used in the same threaded hole
When to Use Grade 5
- General automotive applications
- Machinery and equipment mounting
- Where Grade 8 strength isn't required
- Applications where some ductility is beneficial
- Cost-sensitive applications at volume
Grade 5 is adequate for most fastening applications. It's the default choice when specifications don't require higher strength.
When to Use Grade 8
- High-stress structural connections
- Suspension, steering, and drivetrain components
- Pressure-containing flanges
- When a smaller bolt size is needed for the same strength
- OEM specifications call for Grade 8
Important Considerations
Don't Over-Specify
Using Grade 8 everywhere "just to be safe" has downsides:
- Higher cost for no benefit
- Higher hardness = more brittle = less impact resistance
- More susceptible to hydrogen embrittlement
- May crack rather than deform under unexpected loads
Match the Entire System
- Nut grade should match or exceed bolt grade
- Hardened washers prevent embedment in soft materials
- Thread engagement matters—higher grade won't help stripped threads
Never Substitute Downward
Never replace a Grade 8 bolt with Grade 5 unless you've verified the lower grade meets requirements. If the original designer specified Grade 8, there was a reason.
Metric Equivalents
| SAE Grade | Approximate Metric Class |
|---|---|
| Grade 2 | Class 4.6 |
| Grade 5 | Class 8.8 |
| Grade 8 | Class 10.9 |
Note: These are approximate. Class 10.9 is slightly stronger than Grade 8; Class 8.8 is close to Grade 5.
Torque Differences
For the same bolt size, Grade 8 uses higher torque to achieve proper preload. Using Grade 5 torque values on Grade 8 bolts results in undertightening. Always verify torque specs match the fastener grade.
Common Applications
- Grade 5: Cylinder heads (many), valve covers, brackets, non-critical components
- Grade 8: Rod bolts, main caps, suspension arms, flywheel bolts, wheel studs
The right bolt grade balances strength, ductility, and cost for the specific application. Grade 5 handles most situations; reserve Grade 8 for applications where the extra strength is actually needed.
