Bolt Grades Explained: Grade 2, 5, 8, and Metric Property Classes
Two hex bolts can be the same size, thread, and length and still be nowhere near the same part. The difference is the grade, and it’s stamped right on the head. Put a Grade 2 bolt where a Grade 8 belongs and the joint may hold on the bench, then stretch, loosen, or snap once the machine is under load.
This guide explains how bolt grades work, how to read head markings, how SAE grades compare to metric property classes, and how to choose the right grade. For the broader picture of bolts, screws, anchors, and rivets, see our guide to industrial fastener types.
What a Bolt Grade Tells You
A bolt grade is a strength rating tied to a material and heat-treatment standard. The key numbers are:
- Tensile strength: the load at which the bolt breaks.
- Yield strength: the load at which it permanently stretches.
- Proof load: the load it must withstand without permanent deformation.
Higher grades use harder, heat-treated steels, so they carry more load at the same diameter. In inch sizes, those grades are defined by SAE J429. Metric bolts use ISO 898-1 property classes.
SAE Bolt Grades and Head Markings
| Grade | Head marking | Material | Min. tensile strength (approx.) | Typical use |
|---|---|---|---|---|
| Grade 2 | No marks | Low/medium carbon steel | 74,000 psi (¼”–¾”) | Light-duty, non-critical |
| Grade 5 | 3 radial lines | Medium carbon, quenched & tempered | 120,000 psi (¼”–1″) | General machinery, equipment |
| Grade 8 | 6 radial lines | Medium carbon alloy, quenched & tempered | 150,000 psi | High-stress, heavy equipment |
Grade 2 values drop for larger diameters, and Grade 5 drops to about 105,000 psi above 1 inch. Always check the chart for your size.
Metric Property Classes: 8.8, 10.9, and 12.9
Metric bolts show their property class as a number stamped on the head. The first number × 100 is the approximate tensile strength in MPa. The second number is the ratio of yield to tensile strength.
- 8.8: about 800 MPa (roughly 116,000 psi) and roughly comparable to SAE Grade 5.
- 10.9: about 1,040 MPa (roughly 150,000 psi) and roughly comparable to SAE Grade 8.
- 12.9: about 1,220 MPa (roughly 175,000 psi), common on socket head cap screws in tooling and machine builds.
Those comparisons are close but not exact. When replacing a fastener on equipment built to metric spec, match the property class rather than converting.
Grade 5 vs. Grade 8: Which Do You Need?
Grade 5 is the everyday workhorse for machinery, brackets, guards, and general equipment assembly. It’s strong, a bit more ductile, and more forgiving if the joint sees shock. Grade 8 is for high-stress joints: heavy equipment, suspension and drivetrain components, and connections under high clamp load or vibration.
Stronger isn’t automatically better. Grade 8 is less ductile than Grade 5, so in some shock-load applications it’s more likely to snap than to stretch and give a warning. And a Grade 8 bolt paired with a Grade 2 nut is only as strong as the nut. Match nut grade to bolt grade, and use hardened washers under Grade 8 fasteners.
Not sure which grade your joint calls for? Call (919) 777-9807. We stock Grade 5, Grade 8, metric 8.8 and 10.9, A325 structural bolts, and stainless in Sanford.
Structural and Stainless Bolts
A325 structural bolts (now covered under ASTM F3125) are heavy hex bolts made for structural steel connections. They’re not interchangeable with standard Grade 5 hex bolts in structural work.
Stainless steel bolts (18-8, 304, 316) are chosen for corrosion resistance, not strength. They generally fall below Grade 5 in tensile strength and can’t be hardened by heat treatment. In a washdown area where corrosion is the failure mode, stainless is the right call. On a high-load joint in a dry area, a plated Grade 5 or Grade 8 bolt is stronger.
NC-Specific Considerations
Humidity in NC mills and plants means coating matters as much as grade. A zinc-plated Grade 5 bolt holds up fine indoors in a dry building, while the same bolt near a washdown or in an unheated mill can rust through its plating within a season. For facilities around Raleigh and Greensboro running mixed domestic and imported equipment, keeping both SAE and metric grades on hand avoids the most common maintenance delay we see: the right size in the wrong system.
For joints that vibrate loose even at the correct grade and torque, a threadlocker is often the answer. We cover that in blue vs. red Loctite.
Frequently Asked Questions
Is Grade 5 or Grade 8 stronger?
Grade 8 is stronger, with a minimum tensile strength of about 150,000 psi compared to 120,000 psi for Grade 5 in common sizes. Grade 5 is slightly more ductile, which makes it a good fit for general machinery and shock-load joints.
How do you tell bolt grades apart?
Look at the head. SAE Grade 2 bolts have no markings, Grade 5 bolts have three radial lines, and Grade 8 bolts have six. Metric bolts are stamped with their property class, such as 8.8 or 10.9.
Is Grade 8 or 10.9 stronger?
They are roughly comparable. Metric 10.9 has a minimum tensile strength of about 1,040 MPa (around 150,000 psi), close to SAE Grade 8. Match the specified system and class rather than substituting between them.
Are stainless steel bolts stronger than Grade 8?
No. Common 18-8 and 316 stainless bolts are generally weaker than Grade 5 and well below Grade 8. Stainless is chosen for corrosion resistance, not strength.
Picking the Right Grade
Bolt grades come down to matching strength, ductility, and corrosion resistance to the joint. Read the head, match the nut, and don’t substitute systems. Cruco Supply stocks Grade 5 and Grade 8 hex bolts, metric 8.8 and 10.9, A325 structural bolts, and 304/316 stainless from brands including Brighton-Best, as part of our full anchors and fasteners line. Call (919) 777-9807 or stop by our Sanford location.