How They’re Determined by the SAE, and How They’re Identified by the End User
Automotive bolted joints operate under demanding cyclic loads, high temperatures, and increasingly lightweight designs. As a result, understanding bolt grades-and the standards behind them-is essential for fastener engineers working in OEM, Tier 1, and aftermarket applications. In North America, these grades are governed primarily by the Society of Automotive Engineers (SAE), whose classifications define the mechanical performance of carbon and alloy steel bolts commonly used in vehicles.
This classification is outlined in SAE International’s J429 Mechanical and Material Requirements for Externally Threaded Fasteners. We will be citing that document heavily in this blog.
What Is an SAE Bolt Grade?
An SAE bolt grade is a performance classification that specifies tensile strength, yield strength, hardness, heat-treatment requirements, and material chemistry for automotive fasteners. These performance levels are standardized under SAE J429, ensuring consistent behavior under applied loads. SAE J429 Sections 1-31.
How does the SAE Determine Bolt Grades?
Material Composition
SAE defines allowable compositions for carbon and alloy steels depending on the grade. Higher grades require alloying additions and tighter control of carbon content to meet mechanical properties. Reference: SAE J429: Table 1 (Chemical Composition).
Heat Treating Requirements
Heat treatment processes-quench and temper cycles, austenitizing temperatures, and temper temperatures are defined by the SAE to ensure performance. (SAE J429: Section 3.4).
Mechanical Testing Criteria
Bolt grades are validated through:
- Proof load testing
- Wedge tensile testing
- Axial Tensile Testing
- Yield Strength
- Axial Tensile Strength
- Elongation
- Reduction of Area
(SAE J429: Table 3).
Mechanical Properties by Common SAE Bolt Grades
| US Standard Grades | ||||
| Grade | Typical Use | Min Tensile Strength | Notes | Examples |
| Grade 2 | Low-stress automotive hardware | ~60 ksi | Non-heat-treated; low structural performance |  |
| Grade 5 | Suspension/chassis joints | ~105 ksi | Quenched & tempered; medium strength |  |
| Grade 7 | High Temperature for Exhaust Manifolds and Turbos | ~133 ksi | Quenched & tempered; medium strength |  |
| Grade 8 | Driveline, engine, high-stress joints | ~150 ksi | High-strength alloy steel |  |
On some SAE grades, there are sub-grades within. An example is Grade 5. Within Grade 5, Grade 5.1 is for hex head and sems only; whereas Grade 5.2 is for Bolts and Screws only. Grade 8 components also have two subgrades (8.1 and 8.2, both for bolts and screws only)
End users can identify these variations with the head markings as denoted in the above chart. As you will notice, Grade 5, 5.1 and 5.2 all have three lines. However, Grade 8, 8.1, and 8.2 unfortunately did not follow this convention. In fact, Grade 8.1 studs have no grade mark, per SAE standards compared to six marks for SAE Grade 8 bolts screws and studs as well as 6 marks for SAE Grade 8.2 bolts and screws.
(SAE J429: Pg 117 Table 1).
Metric Fastener Classes
Metric fasteners denote Classes similar to Standard fastener Grades. However, rather than hashes, they simply call them out using the class numbers. You will find the Class numbers and their differences in the chart below.
| US Metric Grades | ||||
| Classes | Typical Use | Min Tensile Strength | Notes | Examples |
| Class 8.8 | Suspension/chassis joints | ~800 MPa | Non-heat-treated; low structural performance |  |
| Class 10.9 | Driveline, engine, high-stress joints | ~1040 MPa | Quenched & tempered; medium strength |  |
| Class 12.9 | Metric Socket Components | ~1220 MPa | Quenched & tempered; medium strength |  |
In addition to these grade and class markings, there are manufacturer identification symbols for traceability in safety-critical applications. (SAE J429: Pg 120, Pg 117 Table 1)
Component Finish or Coating
While coatings do vary greatly among fasteners and components, they are not among the mandated markings by SAE or ISO. As a result, coating type is not a grade identifier and is not regulated by the J429 Quality Requirement. However, some industry practices associate certain finishes with common grades.
Reference: SAE J429: Section 1.3 (Coatings Not Part of Grade Requirements).
Why Bolt Grade Matters in Automotive Applications
Selecting the correct bolt grade impacts:
- Fatigue performance
- Clamp load retention
- Creep and relaxation
- Overload failure modes
- Serviceability and torque requirements
Key Takeaways for Fastener Engineers & End Users
SAE J429 governs most automotive bolt grades used in North America. In addition, grades are defined by material chemistry, heat treatment, and mechanical testing. Grades are clearly identified to end users by head markings for ease of use. Proper grade selection directly affects both joint reliability and long-term durability.
Where does CFI fit in?
For over 30 years, Components for Industry has been a worldwide provider in industrial components such as bolts and other custom fasteners for a wide number of industries like Automotive, Aerospace, Appliance and Heavy Industrial. If you want more information about CFI, our industrial components, our bolts, or our custom design process, contact us at 847-918-0333 or sales@componentsforindustry.com.
- Society for Automotive Engineering. SAE J429: Mechanical and Quality Requirements for Externally Threaded Fasteners. CFR Section(S) 30 CFR 77.403-1(d)(2)(iii)(B). Obtained from: https://law.resource.org/pub/us/cfr/ibr/005/sae.j429.1971.pdf ↩︎
