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Unlock Full Bible — $9.95Bolt Grade Chart
SAE Grade 2/5/8 & Metric Class 4.6–12.9 · Head markings
Bolt Grade Identification Chart
Identify bolt grade by head markings. SAE Grade 2/5/8 and metric Class 4.6/8.8/10.9/12.9 — tensile strength and applications. Never substitute a lower-grade bolt for a higher-grade one on critical diesel engine fasteners.
SAE Bolt Grades
| Grade | Head Marks | Material | Tensile | Use |
|---|---|---|---|---|
| Grade 2 | No marks | Low-carbon steel | 74,000 psi | Non-critical, light-duty |
| Grade 5 | 3 radial lines | Medium-carbon steel, quenched | 120,000 psi | General automotive, brackets |
| Grade 8 | 6 radial lines | Medium-carbon alloy steel | 150,000 psi | High-stress, suspension, drivetrain |
Metric Bolt Classes
| Class | Head Marks | Material | Tensile | Use |
|---|---|---|---|---|
| Class 4.6 | 4.6 | Low-carbon steel | 400 MPa / 58,000 psi | Non-critical, light-duty |
| Class 8.8 | 8.8 | Medium-carbon steel | 800 MPa / 116,000 psi | General engine, brackets |
| Class 10.9 | 10.9 | Alloy steel, quenched | 1040 MPa / 151,000 psi | High-stress, structural |
| Class 12.9 | 12.9 | Alloy steel, heat-treated | 1220 MPa / 177,000 psi | Critical engine fasteners, TTY |
⚠ Critical Safety Rules
Never substitute a lower-grade bolt for a higher-grade one. A Grade 5 bolt in a Grade 8 application will fail under load.
TTY (torque-to-yield) bolts must be replaced after removal — they are permanently stretched during installation and cannot be reused.
Stainless steel bolts are NOT high-strength. Never use stainless in structural or high-stress applications.
Warning: Never substitute a lower-grade bolt in a higher-grade application. TTY bolts must be replaced after every removal.
Need diesel-specific fastener grades and torque specs?
The full Bible includes the diesel fastener database — injector hold-down bolts, turbo mounting bolts, main cap bolts, rod bolts, and head bolt grades by engine family. Includes TTY bolt procedures and VIN-specific torque variants.
Unlock the Full Bible — $9.95Bolt Grade — FAQ
How do I identify a Grade 8 bolt?
Grade 8 bolts have 6 radial lines on the bolt head. Grade 5 bolts have 3 radial lines. Grade 2 bolts have no markings. The more lines, the higher the grade and the stronger the bolt.
What is the difference between Grade 5 and Grade 8 bolts?
Grade 8 bolts have a tensile strength of 150,000 psi versus 120,000 psi for Grade 5. Grade 8 bolts are used in high-stress applications such as suspension, drivetrain, and structural components. Never substitute a lower-grade bolt for a higher-grade one.
What is a Class 10.9 bolt equivalent to in SAE grade?
Metric Class 10.9 bolts (1040 MPa / 151,000 psi tensile strength) are roughly equivalent to SAE Grade 8 in strength. Class 8.8 bolts (800 MPa / 116,000 psi) are roughly equivalent to SAE Grade 5.
Can I reuse TTY (torque-to-yield) bolts?
No. TTY bolts are permanently stretched during installation and must be replaced after removal. They are typically Class 12.9 or equivalent and are used for head bolts, rod bolts, and main cap bolts on diesel engines. Reusing a TTY bolt risks fastener failure.
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