Many engineers and procurement professionals make costly mistakes due to misreading bolt head markings. This article helps you quickly decode these marks, reducing procurement errors and quality issues.
Bolt head markings are standardized symbols—combinations of letters, numbers, and radial lines—that indicate a bolt’s material grade, strength, manufacturing origin, and sometimes even its corrosion resistance. For example, three radial lines mean SAE Grade 5, while “8.8” means 800 MPa tensile strength and 80% yield. Understanding these markings is essential for quality control, procurement, and compliance with international standards.
Let’s look at the most widely used marking systems globally: SAE, ISO, stainless steel grades, manufacturer marks, and surface/coating types.
SAE Grade Markings (Imperial System – U.S.)
SAE standards are used mostly in North America. The grade is shown using radial lines on the head, and they reflect the tensile strength level.
- Grade 2
- Mark: No line on the head
- Material: Low/medium carbon steel
- Tensile strength: Around 400 MPa
- Application: General-purpose, non-critical use
- Grade 5
- Mark: Three radial lines at 12, 4, and 8 o’clock (looks like the Mercedes-Benz logo)
- Material: Medium carbon steel, quenched and tempered
- Tensile strength: Around 830 MPa
- Application: Automotive parts, machinery
- Grade 8
- Mark: Six evenly spaced radial lines
- Material: Medium carbon alloy steel, quenched and tempered
- Tensile strength: Around 1,200 MPa
- Application: High-load conditions like engines or suspension
Metric Bolt Head Markings (ISO / DIN Standards)
Metric bolts, governed by ISO or DIN standards, use a numeric system indicating tensile and yield strength. You’ll typically see two numbers separated by a dot.
Example: 8.8, 10.9, 12.9
- Format Meaning:
- First number × 100 = Minimum tensile strength (MPa)
- Second number × 10 = Percentage of yield to tensile strength
| Marking | Tensile Strength (MPa) | Yield Strength (MPa) | Usage Example |
|---|---|---|---|
| 8.8 | 800 | 640 | General structural use |
| 10.9 | 1,000 | 900 | Automotive suspensions |
| 12.9 | 1,200 | 1,080 | High-performance machinery |
This marking system is internationally used and helps trace performance more easily. It’s shown in most Metric Bolt Identification Charts.
Stainless Steel Bolt Markings
Stainless steel bolts are usually marked by alloy type rather than strength level.
Common Markings:
- 18-8: Contains approximately 18% chromium and 8% nickel – equivalent to 304 stainless steel
- 316: Includes molybdenum. Great for corrosion resistance.
- ASTM F593 Marks:
- F593C – Type 304
- F593D – Type 316
- F593G – Heat-treated, stronger stainless grades
These bolts are ideal for marine, chemical, and high-humidity environments.
Manufacturer Markings
Most bolts include unique identifiers for traceability back to the manufacturer. This helps in quality assurance, recalls, and sourcing.
Examples of Manufacturer Head Markings:
| Manufacturer | Symbol on Bolt Head |
|---|---|
| Fastenal | F |
| Brighton-Best | B |
| Conquest | Shield symbol |
| Wilson-Garner | WG |
Surface Treatment & Material Designations
Some bolt head markings may include secondary marks that indicate:
- Zinc plating: Often seen as
Zor colored tinge - Hot-dip galvanizing: Thicker coatings for external use
- Dacromet or Geomet coatings: Corrosion-resistant ceramic coatings (common in automotive)
Combined markings may look like 10.9 Z, indicating both strength and surface treatment.
Conclusion
Reading bolt head markings accurately ensures the right fastener is used in the right place—preventing failures, improving safety, and saving costs.
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