The grade system behind the radial lines on a bolt head. If someone says “Grade 5” or “Grade 8,” this is the spec they mean.
SAE J429 is the standard that defines the familiar inch-bolt grades — 2, 5, and 8 — setting the material and strength behind each, and the head markings that identify them. Higher grade means a stronger bolt of the same size; the marks let you read which is which at a glance.
Grade 2, 5, 8 inch bolts & screws
Radial lines on the head (none / 3 / 6)
Automotive, machinery, general assembly
| Grade | Head mark | Material | Size | Proof | Yield | Tensile |
|---|---|---|---|---|---|---|
| Grade 2 | none | Low / med carbon | ¼–¾″ | 55 | 57 | 74 |
| Grade 5 | 3 lines | Med carbon, Q&T | ¼–1″ | 85 | 92 | 120 |
| Grade 8 | 6 lines | Med carbon alloy, Q&T | ¼–1½″ | 120 | 130 | 150 |
J429 grades are the everyday inch bolts — Grade 2 for light general work, Grade 5 as the all-around medium-strength standard (automotive, equipment, framing hardware), and Grade 8 where you need real strength (machinery, trailers, high-load joints). It’s a different world from the ASTM structural specs: J429 is about general and automotive fastening, not steel-building connections.
Read the head: a plain head is Grade 2, three radial lines is Grade 5, six lines is Grade 8. The full picture — marks plus proof, yield, and tensile by size — is on the bolt grades & strength chart. When you replace one, match the grade so the new bolt is at least as strong as the old.
Grade 2, 5, and 8 in the sizes and finishes you need — plus the matching nuts and washers. Tell us the grade and size, or bring the bolt and we’ll match it.
