Bolt and Nut Grades Explained: Understanding Strength and Application
When selecting bolts and nuts for any project, understanding grades is crucial. Bolt and nut grades are defined by both the material and the strength, which helps determine how much stress they can handle. Each grade has specific applications, and knowing the differences can help you choose the right fastener for your project.
What is a Bolt and Nut Grade?
Bolt and nut grades are established by various standards organizations and are used to indicate the material and strength of a fastener. Strength requirements are typically based on the material's hardness—stronger fasteners are made from harder materials. Grading systems often include proof load or tensile strength requirements, which verify that the material can withstand the listed forces.
3 Major Standards for Bolt and Nut Grades
There are several organizations that define bolt and nut grading standards, but three of the most widely recognized are SAE, ISO, and ASTM. These organizations create standards for different types of fasteners, including material requirements, strength ratings, and head markings. Below is an overview of how each standard works:
SAE (Society of Automotive Engineers)
SAE is one of the most commonly referenced standards for inch-sized bolts and nuts in North America. SAE J429, for example, defines grades for bolts based on their strength, which is indicated by radial lines on the bolt head.
- SAE J429 Grade 2: Low-strength carbon steel with a strength between 60,000-74,000 psi. Ideal for non-critical applications, such as household repairs.
- SAE J429 Grade 5: Medium-strength carbon or alloy steel with a strength between 105,000-120,000 psi. Used in automotive, military, and heavy machinery.
- SAE J429 Grade 8: High-strength carbon or alloy steel with a strength up to 150,000 psi. These are used in extreme conditions such as heavy machinery and aerospace.
ISO (International Standards Organization)
ISO 898-1 is the most commonly used international standard for metric bolts and nuts, also known as property classes. These fasteners are typically marked with numbers on their heads indicating their strength.
- Property Class 8.8: Medium-strength bolts and nuts comparable to SAE Grade 5, commonly used in automotive and machinery applications.
- Property Class 10.9: High-strength bolts and nuts, comparable to SAE Grade 8, ideal for demanding industrial and automotive applications.
- Property Class 12.9: The strongest available metric bolts and nuts, used for critical applications like securing engine parts.
ASTM (American Society for Testing and Materials)
ASTM sets bolt and nut standards primarily for construction and industrial applications, with ASTM A307 being one of the most recognized bolt standards.
- ASTM A307 Grade A: Low-strength carbon steel bolts with a minimum tensile strength of 60,000 psi, used in non-critical applications.
- ASTM A307 Grade B: Medium-strength bolts used in piping systems and other flanged joints, with a tensile strength up to 100,000 psi.
Comparison of Bolt and Nut Grades by Standard
The table below provides a quick reference for comparing different bolt and nut grades across SAE, ISO, and ASTM standards, as well as their material types and common applications.
| Grade/Class | Strength (psi) | Common Applications | Standard | Material |
|---|---|---|---|---|
| SAE Grade 2 | 60,000-74,000 | Non-critical applications (household repairs) | SAE J429 | Carbon Steel |
| SAE Grade 5 | 105,000-120,000 | Automotive, military, machinery | SAE J429 | Carbon or Alloy Steel |
| SAE Grade 8 | Up to 150,000 | Heavy machinery, aerospace | SAE J429 | Carbon or Alloy Steel |
| ISO Class 8.8 | Comparable to Grade 5 | Automotive, machinery | ISO 898-1 | Carbon or Alloy Steel |
| ISO Class 10.9 | Comparable to Grade 8 | Heavy machinery, industrial | ISO 898-1 | Carbon or Alloy Steel |
| ISO Class 12.9 | Strongest metric | Critical applications, engines | ISO 898-1 | Alloy Steel |
| ASTM A307 Grade A | 60,000 | Non-critical construction | ASTM A307 | Carbon Steel |
| ASTM A307 Grade B | Up to 100,000 | Piping, flanged joints | ASTM A307 | Carbon Steel |
| Grade A Nuts | Exceeds Grade 2 | Light-duty, general use | SAE | Carbon Steel |
| Grade B Nuts | Similar to Grade 5 | Medium-strength applications | SAE | Carbon Steel |
| Grade G Nuts | Similar to Grade 8 | Structural, ASTM A325 bolts | SAE | Carbon Steel |
| Stainless Steel A2-70 | 74,000 | Corrosion-resistant applications | ISO | Stainless Steel |
| Stainless Steel A4 | 180,000 | Marine, chemical environments | ISO | Stainless Steel |
| Titanium Grade 5 | 138,000 | Aerospace, medical devices | ISO | Titanium |
US Nut Grades Explained
Like bolts, nuts are graded according to their material strength, and it’s important to match the grade of the nut with the grade of the bolt for optimal performance. Below are some common US nut grades:
- Grade A Nuts: Strength exceeds Grade 2.
- Grade B Nuts: Strength similar to Grade 5.
- Grade 2 Nuts: Low strength, suitable for non-critical applications.
- Grade 2H Nuts: Strength similar to Grade 5. Used with ASTM A325 structural bolts.
- Grade C Nuts: Strength exceeds Grade 5. Used in construction applications.
- Grade G Nuts: Strength similar to Grade 8. Used with ASTM A325 structural bolts.
- Grade 8 Nuts: High strength, used in heavy-duty applications such as construction machinery.
Metric Nut Grades Explained
In metric applications, nut strength is measured by property classes. Here are some common metric nut grades and their US equivalents:
- Metric Class 4: Similar to Grade 2, these nuts are low-strength and used in non-critical applications.
- Metric Class 8.8: Equivalent to Grade 5, these nuts are used in medium-strength applications such as automotive and light industrial.
- Metric Class 10.9: Comparable to Grade 8, these high-strength nuts are used in demanding applications.
- Metric Class 12.9: The highest metric class, these nuts exceed Grade 8 strength and are used in critical, high-stress applications like engine components.
Applications of Different Nut Grades
Pairing nuts with the correct grade of bolt is essential to ensure the connection can handle the required load. Using a lower-grade nut with a higher-grade bolt could result in the nut stripping or failing, while using a stronger nut with a weaker bolt may not provide any additional benefits.
- Grade A Nuts: Non-critical home and DIY projects.
- Grade B Nuts: Medium-strength applications such as automotive and machinery.
- Grade G & Grade 8 Nuts: Heavy-duty applications in construction and high-stress environments.
- ASTM A325 Nuts: Structural applications involving heavy loads and steel connections.
- Metric Class 8.8 and 10.9: Automotive and industrial machinery applications requiring medium to high strength.
- Metric Class 12.9: Critical, high-stress environments such as engine assembly and aerospace.
FAQ About Bolt and Nut Grades
What is the difference between Grade 5 and Grade 8 bolts?
Grade 5 bolts are medium-strength fasteners with tensile strength between 105,000-120,000 psi, while Grade 8 bolts are high-strength with tensile strength up to 150,000 psi.
Can I use Grade 8 nuts with Grade 5 bolts?
It’s generally recommended to match the grade of your nuts and bolts. Using a Grade 8 nut with a Grade 5 bolt may not offer any performance benefits and could result in failure if the bolt fails under load.
What is the strongest nut grade?
Metric Class 12.9 nuts are the strongest, followed by Grade 8 in US standards. These are used in critical applications like engine components and aerospace.
How do I know what grade my bolt is?
Most bolts have head markings that indicate their grade. For SAE bolts, radial lines on the bolt head indicate the grade. For ISO bolts, raised or stamped numbers indicate the property class.
Can I mix metric and SAE bolts and nuts?
Mixing metric and SAE fasteners is generally not recommended due to differences in size, thread pitch, and strength ratings. Always use matching nuts and bolts for the best performance.
Explore Our Selection of Bolts and Nuts
At Monster Bolts, we offer a wide selection of high-quality fasteners. Whether you need high-strength Grade 8 bolts, 18-8 stainless steel bolts, or a variety of SAE and metric nuts, we have the right fasteners for your project.
- Shop Grade 8 Bolts
- Shop 18-8 Stainless Steel Bolts
- Shop SAE Nuts
- Shop Metric Nuts
- Shop Metric Class 12.9, Titanium Grade 5, and Stainless A2/A4 Screws
For more information on fasteners, be sure to check out our other blog posts or reach out to our team of fastener experts.