Why Grade Matters
Bolt grade indicates strength. Using the wrong grade can lead to failure under load—a safety hazard. Fortunately, SAE bolt grades are marked directly on the head, so you can identify them at a glance.
SAE Grade Markings (Radial Lines)
SAE grades are marked with radial lines (dashes extending from center to edge) on hex bolt heads:
| Grade | Head Marking | Tensile Strength | Common Use |
|---|---|---|---|
| Grade 2 | No lines (plain) | 74,000 PSI | Light duty, non-critical |
| Grade 5 | 3 radial lines | 120,000 PSI | Automotive, construction |
| Grade 8 | 6 radial lines | 150,000 PSI | Heavy equipment, critical |
Memory trick: Subtract 2 from the number of lines to get the grade. 3 lines = Grade 5, 6 lines = Grade 8.
Metric Class Markings (Numbers)
Metric bolts use a two-digit number separated by a decimal point:
| Class | Head Marking | Tensile Strength | SAE Equivalent |
|---|---|---|---|
| Class 4.6 | 4.6 | 400 MPa | ~Grade 2 |
| Class 8.8 | 8.8 | 800 MPa | ~Grade 5 |
| Class 10.9 | 10.9 | 1040 MPa | ~Grade 8 |
| Class 12.9 | 12.9 | 1220 MPa | >Grade 8 |
The first number × 100 = tensile strength in MPa. The decimal × first number × 100 = yield strength.
Stainless Steel Markings
Stainless bolts don't use the SAE grade system. Look for:
- 18-8 or A2: Standard stainless (304 alloy)
- 316 or A4: Marine-grade stainless
- F593: ASTM specification for stainless bolts
Stainless is weaker than Grade 5/8 steel but offers corrosion resistance. Never substitute stainless for high-strength applications without engineering approval.
Socket Head Cap Screws
Socket head caps don't have room for radial lines. Instead:
- No marking: Typically alloy steel (similar to Grade 8)
- "A2" or "A4": Stainless steel
Assume unmarked socket caps are high-strength unless specified otherwise.
Carriage Bolts and Others
Carriage bolts (round head) can't display radial lines. Grade is typically indicated by:
- Manufacturer markings on the dome
- Packaging or bin labeling
- Color coding (varies by manufacturer)
When in Doubt
- Check the original packaging or bin label
- Ask your supplier (that's us!)
- Assume lower grade for safety
- Replace with a known grade if critical