1. Introduction
Head markings on bolts, nuts and washers are the most authoritative standard for strength judgment, quality inspection and engineering acceptance. Many overseas buyers judge fastener quality only by appearance or coating color, causing common mistakes such as mismatched strength grades, mixed high-strength and ordinary fasteners, and unqualified bolt-nut matching. These errors lead to thread slipping, bolt fracture, deformation and project rework.
Each fastener grade has standardized marking rules for metric carbon steel, stainless steel and imperial standard parts. This article systematically explains how to read bolt, nut and washer surface markings, providing complete sourcing and inspection guidelines for global purchasers.
2. Metric Carbon Steel Bolt Marking Guide (4.8 / 8.8 / 10.9 / 12.9)
Metric carbon steel bolts adopt decimal grade marking, directly representing tensile strength level.
Grade 4.8: Usually blank or only brand logo, low strength, for light fixing and indoor decoration.
Grade 8.8: Clearly marked “8.8”, medium-high strength, widely used in mechanical equipment and general steel structures.
Grade 10.9: Marked “10.9”, high tensile and fatigue resistance, suitable for heavy machinery, flange and vibrating equipment.
Grade 12.9: Marked “12.9”, ultra-high strength for heavy industry, mold and high-load precision equipment.
3. Stainless Steel Bolt Marking Explanation (A2-70 / A4-80)
Stainless steel bolts use A-grade marking instead of decimal numbers.
A2-70 (304 Stainless Steel): General anti-rust grade for ordinary outdoor and corrosion-resistant projects.
A4-80 (316 Stainless Steel): Higher strength and salt spray resistance, specially used for coastal, marine and chemical corrosive environments.
4. Metric Nut Strength Marking & Matching Rule
Nuts use integer grade marks without decimals. The core rule: Nut grade ≥ Bolt grade.
Grade 4 matches 4.8 bolts; Grade 8 matches 8.8 bolts; Grade 10 matches 10.9 bolts; Grade 12 matches 12.9 bolts. Mismatched grades will cause thread slipping and nut fracture.
5. Washer Strength & Identification Standard
Ordinary flat washers and spring washers have no marks. High-strength washers are printed with HV200 / HV300 hardness codes. High-tensile bolt assemblies must use matched high-strength washers to avoid collapse and loosening.
6. Imperial Bolt Grade Identification (Grade 5 / Grade 8)
US standard bolts use line marks instead of numbers:
2 lines = Grade 5 medium strength
6 lines = Grade 8 high strength
7. Fastener Grade Matching Table
Bolt Grade | Matched Nut Grade | Application |
4.8 | 4 | Light indoor fixing |
8.8 | 8 | General machinery & steel structure |
10.9 | 10 | Heavy equipment & flange connection |
12.9 | 12 | Ultra-high load heavy industry |
A2-70 | A2 | Ordinary anti-corrosion projects |
A4-80 | A4 | Marine & chemical corrosive environment |
8. Common Sourcing Mistakes
Mixing different strength grades causes structural failure. Using low-grade nuts for high-strength bolts leads to thread damage. Confusing A2 and A4 stainless grades causes rusting complaints in coastal projects.
9. Conclusion
Fastener surface markings are the most reliable standard for strength classification. Buyers must match bolt, nut and washer grades strictly according to printed marks to ensure engineering safety and avoid after-sales risks.
