Overview
Grade 5 and Grade 8 are the two most common high-strength bolt grades in the SAE (Society of Automotive Engineers) system. Understanding when to use each is critical for safety and cost-effectiveness.
Quick Comparison
| Property | Grade 5 | Grade 8 |
|---|---|---|
| Material | Medium Carbon Steel | Medium Carbon Alloy Steel |
| Tensile Strength | 120,000 PSI min | 150,000 PSI min |
| Yield Strength | 92,000 PSI min | 130,000 PSI min |
| Head Marking | 3 radial lines | 6 radial lines |
| Relative Cost | Lower | Higher (~20-40% more) |
How to Identify Bolt Grade
Look at the head of the bolt for radial lines (marks extending from the center to the edge):
- No lines: Grade 2 (low carbon steel)
- 3 lines: Grade 5 (medium strength)
- 6 lines: Grade 8 (high strength)
When to Use Grade 5
- Automotive body and frame applications
- Construction and structural connections (per code)
- Agricultural equipment
- General machinery
- Most applications where a "strong bolt" is needed
Grade 5 is sufficient for 90%+ of applications. It's the standard high-strength bolt for automotive, construction, and general industrial use.
When to Use Grade 8
- Heavy equipment and machinery
- High-stress suspension components
- Critical structural connections
- Applications with cyclic loading
- Anywhere the specifications require it
Use Grade 8 when specified or when the application demands the highest strength. Don't over-specify—Grade 8 costs more and the extra strength may not be needed.
Important: Match Grade of Nut to Bolt
Always use a nut of equal or higher grade than the bolt. A Grade 5 nut with a Grade 8 bolt will fail at the nut before the bolt reaches its rated strength.
- Grade 5 bolt → Grade 5 or Grade 8 nut
- Grade 8 bolt → Grade 8 nut only
Common Mistakes
- Mixing grades: Using a Grade 5 nut with a Grade 8 bolt
- Over-torquing: Higher grade doesn't mean tighten more—follow torque specs
- Reusing stretch bolts: Some critical bolts are designed for one-time use
- Ignoring finish: Hot-dip galvanized bolts may have lower effective grade due to threading