Understanding Bolt Head Markings and Their Significance

Bolt head markings are essentially a mix of letters and numbers that assist in identifying the specific fastener required. By decoding these markings, one can discern the manufacturer, the material composition, and the tensile strength the bolt is capable of withstanding.

Standards and specifications for various grades of bolts and fasteners are provided by numerous organizations, with the SAE (Society of Automotive Engineers) standard and the metric system being among the most prevalent.

Exploring bolt head markings further can significantly enhance the selection process for fasteners, ensuring more informed decisions for projects.

Bolt Head Markings and Grades

The choice between metric and standard formats largely depends on the specific requirements or preferences of a project. Metric fasteners are generally preferred for products intended for the European market, though it is crucial not to interchange these standards due to differences in threads, measurement units, and other specifications.

The Fasteners Quality Act mandates that all fasteners bear a unique manufacturer symbol for traceability, allowing for accountability in case of defects and ensuring consumer confidence in the product quality.

Each fastener grade or class comes with its own set of marking requirements that manufacturers must comply with.

English/Imperial/Standard/SAE Bolt Head Markings

Bolt grades in this category range from grade 2, the lowest, to grade 8, the highest, indicating tensile strength through radial lines or other symbols.

  • Grade 2 Bolt

Marked solely with the manufacturer’s symbol, these bolts are made from low-carbon steel and are not heat-treated due to their low strength. They are the most cost-effective option for light-duty applications.

  • Grade 5 Bolt

Featuring three radial lines in addition to the manufacturer’s symbol, grade 5 bolts are manufactured from medium-carbon steel and undergo case hardening. This makes them hard on the exterior but relatively soft internally, suitable for the automotive industry.

  • Grade 8 Bolt

With six radial lines for identification, grade 8 bolts are fully heat-treated for maximum strength. These bolts are ideal for heavy-duty applications requiring high structural integrity.

  • 18-8 Stainless Steel Bolt

Composed of 17%-19% chromium and 8%-13% nickel, the markings on these bolts can vary as there is no universal standard.

The ASTM standards also include:

  1. A307 Grades A and B
  2. A193 Grade B7
  3. A354 Grades BC and BD
  4. Metric Bolt Head Markings

Set by ISO standards, metric bolts use two numbers separated by a dot to indicate the class, simplifying the process for applications requiring a broad range of sizes. These bolts range from class 4.6 to 12.9, with the most common being:

  • Class 8.8 Bolts

Constructed from medium-carbon steel, these bolts demonstrate nominal tensile strength of 800MPa and yield strength of 640MPa for a 16mm diameter bolt. They are durable but not suited for extreme temperatures.

  • Class 10.9 Bolts

Made from alloy steel and heat-treated, bolts in this class have a minimum tensile strength of 1040MPa, making them suitable for automotive assembly applications.

  • Class 12.9 Bolts

These are the strongest bolts, made from heat-treated alloy steel, with a tensile strength of 1220MPa. They are commonly used in demanding sectors like shipbuilding, oil and gas, and chemical industries.

  • A-2 Stainless Steel Bolts

These bolts are similar to the SAE stainless steel variants, with variable markings.

When selecting a bolt, it’s crucial to consider not only the size but also the proof load, yield strength, and tensile strength. For all your fastening requirements, Big Bolt is your go-to source for high-quality fasteners, including custom orders, ready to meet your project deadlines. Reach out for a quotation on your next project.

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