Deciphering Bolt Grades: A Guide to Bolt Grading Systems

To embark on construction or manufacturing projects successfully, a fundamental understanding of bolt grades proves to be indispensable. These grades serve as a clear indicator of a bolt’s strength and its suitability for specific applications. To discern these grades, one need only observe the markings etched onto the bolt’s head. In our pursuit to demystify this intricate subject, we present this illuminating article. 

Globally, various bolt grading systems find application, including SAE (Society of Automotive Engineers), ASTM (American Society for Testing and Materials), and ISO (International Organization for Standardization).

SAE Grades

An indispensable role in fortifying structures is played by bolts. The application of SAE (Society of Automotive Engineers) grade bolts is widespread, mainly due to their distinctive tensile strength properties. Familiarizing oneself with these grades becomes imperative when it comes to selecting the most suitable bolt for a particular task. So, let us delve deeper into the realm of SAE grades

SAE Grades at a Glance

Grade	Head Markings	Tensile Strength (psi)
Grade 2	No head markings	74,000 – 92,000
Grade 5	Three radial lines	120,000
Grade 8	Six radial lines	150,000

Grade 2

• Grade 2 bolts are the most basic and commonly used hardware grade steel bolts. They are readily available and used for various general applications where high tensile strength is not a critical requirement;
• Tensile strength ranges from 74,000 to 92,000 pounds per square inch (psi), making them suitable for tasks where moderate strength is sufficient;
• These bolts do not feature any head markings, making them easily identifiable.

Grade 5

• Grade 5 bolts are a step up from Grade 2 in terms of tensile strength and are recognizable by the presence of three radial lines on the head;
• Their tensile strength is about 120,000 psi, making them significantly stronger than Grade 2 bolts. This strength is crucial in automotive applications where bolts need to withstand higher stress levels;
• Due to their increased strength, Grade 5 bolts are commonly used in automotive manufacturing, especially in engine components and chassis assemblies.

Grade 8

• Grade 8 bolts are the highest strength bolts among the SAE grades and are easily distinguishable by the six radial lines on the head;
• With an impressive tensile strength of 150,000 psi, Grade 8 bolts are engineered to handle heavy-duty applications that demand exceptional strength and reliability;
• Their durability makes them ideal for critical automotive and engineering tasks, such as securing suspension systems and heavy machinery parts.

When choosing the appropriate bolt grade for your project, it’s essential to consider the specific requirements and loads the bolts will be subjected to. Using the wrong grade can compromise the structural integrity and safety of the assembly. Therefore, always consult the engineering specifications and guidelines for the project and opt for the SAE grade that best suits your needs.

ASTM Grades

ASTM (American Society for Testing and Materials) bolts are the backbone of secure and reliable connections. Understanding the different grades is crucial for ensuring the structural integrity and safety of the projects. Let’s delve into the world of ASTM bolts:

ASTM Bolts Grades and Tensile Strengths

Grade	Tensile Strength (psi)
A307	60,000
A325	105,000
A490	150,000

Grade A307

• Comparable to SAE Grade 2 bolts, Grade A307 bolts are a common choice for various general-purpose construction applications where moderate strength is sufficient;
• Boasting a tensile strength of 60,000 pounds per square inch (psi), these bolts are reliable for tasks that don’t demand the highest levels of strength;
• Their versatility and cost-effectiveness make them popular for fastening non-critical components in construction projects.

Grade A325

• For structural steel applications, Grade A325 bolts step up the strength considerably. These bolts are designed to withstand heavy loads and provide robust connections;
• With a mighty tensile strength of 105,000 psi, these bolts ensure the stability of essential structural elements, such as beams, columns, and connections in bridges and buildings;
• Due to their high strength, these bolts feature a heavy hexagonal head to provide a larger bearing surface and enhance load-bearing capacity.

Grade A490

• At the top of the ASTM bolt hierarchy, Grade A490 bolts are the go-to choice for high-strength structural applications where maximum strength and reliability are paramount;
• Exhibiting an impressive tensile strength of 150,000 psi, Grade A490 bolts can handle intense loads and harsh environmental conditions with ease;
• Like their SAE counterpart, Grade 8 bolts, these structural powerhouses are vital in critical engineering projects, including bridges, high-rise buildings, and other heavy construction works.

When working with ASTM bolts, it is essential to adhere to the manufacturer’s guidelines and follow proper installation procedures to ensure optimal performance and safety. Additionally, understanding the specific requirements of the project will help in selecting the appropriate ASTM grade for the job.

ISO Grades

ISO grading system sets itself apart by using property classes to denote the strength of bolts. These property classes offer a diverse range of tensile strengths, catering to a wide array of applications. Let’s embark on a captivating journey to discover the power of ISO bolt classes:

ISO Bolt Classes and Tensile Strengths

Class	Tensile Strength (psi)
4.6	58,000
8.8	116,000
10.9	145,000

Class 4.6

• Like a reliable companion, Class 4.6 bolts offer moderate strength suitable for various general-purpose applications. These bolts are akin to SAE Grade 2 bolts and ASTM A307 bolts, making them accessible and widely used in a variety of projects;
• Boasting a tensile strength of 58,000 pounds per square inch (psi), these bolts provide a sturdy connection without being overly rigid. They find their place in projects where high strength is not the primary requirement, and cost-effectiveness is essential;
• Class 4.6 bolts are commonly used in everyday construction tasks, fastening non-critical components, and providing secure connections for light to moderate loads.

Class 8.8

• Stepping up the strength game, Class 8.8 bolts are comparable to SAE Grade 5 bolts and ASTM A325 bolts, offering a higher tensile strength to tackle more demanding applications;
• With a robust tensile strength of 116,000 psi, these bolts are ideal for structural steel applications and heavy-duty projects. They are trusted to hold together vital components in bridges, buildings, and machinery;
• The versatility of Class 8.8 bolts makes them a popular choice for various industries, where reliable connections are vital for the safety and integrity of the structures.

Class 10.9

• At the pinnacle of the ISO grading system, Class 10.9 bolts reign supreme in the realm of high-strength fasteners. These bolts can take on the toughest challenges and withstand the most rigorous conditions;
• Comparable to the might of SAE Grade 8 bolts and ASTM A490 bolts, Class 10.9 bolts boast an impressive tensile strength of 145,000 psi. They provide unmatched stability and durability, making them indispensable for critical engineering projects;
• From heavy machinery to high-rise buildings, Class 10.9 bolts are the backbone of structures that demand unwavering strength and reliability.

When working with ISO bolts, it’s crucial to consider the specific requirements and loads your project will encounter. Selecting the appropriate class will ensure the safety and success of your endeavors.

Head Markings and Bolt Identification

Before we embark on deciphering the mysterious head markings, let us briefly revisit the grade markings that adorn bolts. These markings, usually imprinted on the bolt’s head, signify the strength and material properties of the bolt. They allow engineers, builders, and enthusiasts to make informed decisions about which bolts to use for specific applications.

Grade	Marking	Strength
Grade 2	No markings	Low strength
Grade 5	Three lines	Medium strength
Grade 8	Six lines	High strength

Once the code of grade markings has been cracked, it’s time to unravel the mysteries of manufacturer marks. These peculiar symbols or letters, etched discreetly onto the bolt’s head, may appear perplexing at first, but they hold valuable information for those who seek to know more.

• Manufacturer Identification: Just as fingerprints are unique to individuals, so too are these manufacturer marks exclusive to the companies that forge the bolts. These marks serve as a signature, allowing experts to trace the bolt back to its birthplace. For instance, “ABC” might represent the prestigious ABC Bolt Company, while a cryptic symbol could signify a lesser-known, specialized manufacturer;
• Date and Batch Codes: Some manufacturer marks are akin to time capsules, revealing the bolt’s date of production and the batch to which it belongs. A series of numbers, letters, or a combination of both may conceal this temporal information. Thus, the humble bolt becomes a time traveler, holding the secrets of its birth within its unassuming head;
• Quality Control and Traceability: In the intricate world of engineering, quality control is paramount. Manufacturer marks not only aid in identifying the origin of the bolt but also serve as a crucial component in ensuring traceability. In the event of any issues or defects, these marks can be used to recall specific batches and investigate potential problems, ensuring safety and reliability in construction and machinery.

Conclusion

To ensure you choose the appropriate hardware for your project, it is vital to grasp the significance of bolt grades and their corresponding markings. Whether you are a seasoned engineer or a DIY enthusiast, acquainting yourself with these grades holds the key to the longevity and safety of your work. Make sure to watch this informative video that explains bolt grades, providing you with further insights into this domain:

Remember, while the bolt’s strength is undeniably important, it is equally crucial to consider the materials being joined, the prevailing environmental conditions, and the specific requirements of your project. The decision doesn’t solely revolve around opting for the highest grade; rather, it entails selecting the grade that perfectly matches the unique needs of your project.

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