Fastening Know-How
Grade 8 Bolts: SAE J429 Strength, Markings and Specs
A Grade 8 bolt is the highest common SAE J429 strength grade: medium carbon alloy steel, quenched and tempered, with a minimum tensile strength of 150,000 psi from 1/4 to 1-1/2 inch. You can recognise it by six radial lines on the head.
What a Grade 8 Bolt Is
SAE J429 sets the mechanical and material requirements for inch-series bolts, screws and studs used in automotive and general industrial work. Grade 8 sits above Grade 5 and is chosen where a joint needs more clamp load in the same size, or the same clamp load from a smaller bolt.
For all SAE grades side by side, see the bolt grade chart.
SAE J429 Grade 8 Specifications
Minimum mechanical properties and identification for Grade 8.
| Property | Grade 8 |
|---|---|
| Size range | 1/4" to 1-1/2" |
| Proof load, min | 120,000 psi |
| Yield strength, min | 130,000 psi |
| Tensile strength, min | 150,000 psi |
| Material | Medium carbon alloy steel, quenched and tempered |
| Head marking | Six radial lines |
Sources, matched value by value: California Fastener, SAE J429 specification library, and American Fastener Technologies, ASTM, SAE and ISO grade markings chart (both accessed 2 Oct 2026). Core hardness is withheld: only one of the two sources lists it. This is a summary; check the current edition of SAE J429 before relying on it for design.
Grade 8 vs Grade 5 vs ASTM F3125 Grade A490
Grade 8 and A490 share a 150 ksi minimum tensile strength, but they are different products for different jobs.
| SAE J429 Grade 5 | SAE J429 Grade 8 | ASTM F3125 Grade A490 | |
|---|---|---|---|
| Size range | 1/4" to 1-1/2" | 1/4" to 1-1/2" | 1/2" to 1-1/2" |
| Tensile strength, min | 120,000 psi (to 1"); 105,000 psi (over 1") | 150,000 psi | 150 ksi |
| Tensile strength, max | Not listed in our sources | Not listed in our sources | 173 ksi |
| Yield strength, min | 92,000 psi (to 1"); 81,000 psi (over 1") | 130,000 psi | Withheld |
| Head marking | Three radial lines | Six radial lines | "A490" plus the manufacturer mark |
| Typical use | General automotive and machinery | High-load automotive and machinery joints | Pretensioned steel-to-steel structural connections |
Grade 5 and Grade 8 values: California Fastener and American Fastener Technologies (accessed 2 Oct 2026). A490 values: Research Council on Structural Connections, Specification for Structural Joints Using High-Strength Bolts (2020), and Portland Bolt, F3125 grades (accessed 2 Oct 2026). A490 minimum yield is withheld because the two sources do not both state it for F3125. An older chart lists a 170 ksi maximum for the withdrawn A490 standard; we show the 173 ksi stated for F3125.
Choosing Grade 8 Over Grade 5
Grade 8 gives about 25 percent more minimum tensile strength than Grade 5 up to 1 inch, and holds 150,000 psi across the full size range, while Grade 5 drops above 1 inch. That makes it the usual choice for suspension, drivetrain and heavy equipment joints that see high static loads.
Higher strength is not always better. Harder bolts are more sensitive to notches and to hydrogen embrittlement from some plating processes, so a cyclically loaded joint or a part that will be electroplated needs an engineering review rather than an automatic upgrade.
Grade 8 is not a structural steel bolt. Pretensioned steel connections designed to the RCSC specification use ASTM F3125 bolts. See ASTM F3125 structural bolts.
Nuts, Washers and Torque
Match the nut to the bolt so the bolt, not the nut thread, is the weaker part of the joint, and use hardened washers under the head and nut where the bearing surface needs it.
For tightening values, use the torque and preload calculator or the Grade 8 bolt torque chart, and for a wider comparison of structural and SAE grades read A325, A490, F3125, B7 and SAE Grade 5 and 8 compared.
Grade 8 Bolt FAQ
How strong is a Grade 8 bolt?
SAE J429 Grade 8 has a minimum tensile strength of 150,000 psi, a minimum yield strength of 130,000 psi and a proof load of 120,000 psi, from 1/4 to 1-1/2 inch.
How do I identify a Grade 8 bolt?
Look for six radial lines on the head, alongside the manufacturer mark. Grade 5 has three lines.
Is Grade 8 stronger than Grade 5?
Yes. Grade 8 has a 150,000 psi minimum tensile strength against 120,000 psi for Grade 5 up to 1 inch, and 105,000 psi for Grade 5 above 1 inch.
Can I use Grade 8 bolts for structural steel connections?
Not where the RCSC specification applies. Those joints use ASTM F3125 bolts such as Grade A325 or A490. See ASTM F3125.
Is a Grade 8 bolt the same as an A490 bolt?
No. Both have a 150 ksi minimum tensile strength, but A490 is a heavy hex structural bolt with a 173 ksi maximum and its own marking, testing and installation rules.
Sourcing Grade 8 Bolts?
Tell us the size, length, finish and quantity. We supply graded bolts with certification, on one order or a managed programme.