Grade 5, Grade 8, Class 8.8 and 10.9: How to Choose the Right Bolt
It is one of the most common questions in purchasing and maintenance: Grade 5 or Grade 8? And how do they compare with metric classes 8.8 or 10.9? The wrong choice can mean a joint that loosens, a bolt that breaks or, the other way around, paying for strength you don’t need. Here is how to read each system and when to use each grade.
Two systems: SAE (inch) and ISO (metric)
- SAE J429 defines grades for inch fasteners: Grade 2, Grade 5 and Grade 8 are the most common.
- ISO 898-1 defines property classes for metric carbon and alloy steel fasteners: 4.6, 8.8, 10.9 and 12.9, among others.
Both systems measure the same things (tensile strength and yield strength), but with different numbers.
How to read a metric property class
In an ISO class, the first number times 100 gives the nominal tensile strength in megapascals (MPa), and the second number gives the yield strength as a fraction of that value:
- 8.8: 800 MPa nominal tensile; yield at 80%, i.e. 640 MPa.
- 10.9: 1,000 MPa nominal tensile; yield at 90%, i.e. 900 MPa.
- 12.9: 1,200 MPa nominal tensile; yield of 1,080 MPa.
Comparison table
| Grade / class | Standard | Approx. min. tensile | Head marking | Typical use |
|---|---|---|---|---|
| Grade 2 | SAE J429 | 74 ksi (≈510 MPa), up to 3/4″ | No marks | General purpose, light loads |
| Grade 5 | SAE J429 | 120 ksi (≈830 MPa), up to 1″ | 3 radial lines | Automotive, machinery, light structures |
| Grade 8 | SAE J429 | 150 ksi (≈1,030 MPa) | 6 radial lines | Suspension, heavy machinery, high vibration |
| Class 8.8 | ISO 898-1 | 800 MPa nominal | “8.8” | Close metric counterpart to Grade 5 |
| Class 10.9 | ISO 898-1 | 1,000 MPa nominal | “10.9” | Close metric counterpart to Grade 8 |
| Class 12.9 | ISO 898-1 | 1,200 MPa nominal | “12.9” | Tooling, high-strength socket head cap screws |
Exact values vary with diameter; always check the current standard for your size.
What about the A325 structural bolt?
Structural steel connections use structural bolts such as the A325 (now part of ASTM F3125). Its strength is comparable to Grade 5, but its heavy hex head, thread length and use with a hardened nut and washer are designed for bolted structural joints.
How to choose
- Start with the load and joint type. If the joint works in tension or shear with high loads or vibration, go up a grade. For light fastening, Grade 2 or 5 is usually enough.
- Don’t mix systems. Don’t pair a metric bolt with an inch nut, or a Grade 8 bolt with a lower-grade nut: the nut must match the bolt’s strength.
- Higher grade is not always better. Higher grades are harder and less ductile; in some applications with impact loads or acid-based coating processes, the risk of hydrogen embrittlement increases. That is why coating and baking (de-embrittlement) matter.
- Check head markings on receipt. The 3 or 6 lines, or the stamped class, plus the manufacturer’s mark, are your first inspection.
- Ask for material and test certificates. For critical applications, request tensile, hardness and chemical composition reports by lot.
APISA Fasteners manufactures fasteners in SAE grades and metric classes, such as Grade 5 hex bolts, as well as custom made-to-drawing parts. If you’re unsure which grade your application needs, contact us.
Frequently asked questions
What is the difference between Grade 5 and Grade 8?
Grade 8 has higher tensile strength (about 150 ksi versus 120 ksi for Grade 5). It is identified by 6 radial lines on the head; Grade 5 has 3.
Is an 8.8 bolt the same as Grade 5?
They are close in strength but not identical or interchangeable: one is metric (ISO 898-1) and the other inch (SAE J429), with different threads.
What do the numbers in a metric class like 10.9 mean?
The 10 means 1,000 MPa nominal tensile strength, and the .9 means yield strength is 90% of that value, i.e. 900 MPa.
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