Understanding Bolt Grades and Material Markings
Decode the markings on bolt heads to understand their strength, material, and proper applications.
The marks on a bolt head aren’t decoration — they’re a code for how much load the fastener can take. Read them right and you’ll match strength to the job every time; read them wrong and a joint can work loose or snap. Whether you’re hanging a shelf or torquing suspension bolts on a 2022 Ford F-150, here’s how to decode the head.
Why the grade matters
The grade sets the clamping force a bolt can hold before it stretches or breaks.
SAE (inch) grades
Count the radial lines on the head: no lines, three, or six.
| SAE grade | Head marking | Tensile strength | Typical use |
|---|---|---|---|
| Grade 2 | No radial lines | 74,000 psi | Light-duty, non-structural |
| Grade 5 | 3 radial lines | 120,000 psi | General automotive & machinery |
| Grade 8 | 6 radial lines | 150,000 psi | High-stress, critical joints |
Metric classes
Metric heads skip the lines and stamp the class number straight on.
| Metric class | Head stamp | Tensile strength | Typical use |
|---|---|---|---|
| Class 4.6 | 4.6 | 400 MPa (58,000 psi) | Light-duty |
| Class 8.8 | 8.8 | 800 MPa (116,000 psi) | General-purpose (like Grade 5) |
| Class 10.9 | 10.9 | 1040 MPa (150,000 psi) | High-strength (like Grade 8) |
| Class 12.9 | 12.9 | 1220 MPa (177,000 psi) | Maximum strength, alloy steel |
Notice the overlap: a Grade 8 bolt and a metric class 10.9 are near-equivalents in strength.
Match the grade to the job
Meet the spec or beat it — never drop under it on a load-bearing joint.
| If you need… | Reach for | Examples |
|---|---|---|
| Light-duty hold | Grade 2 / class 4.6 | Brackets, covers, furniture |
| General strength | Grade 5 / class 8.8 | Most automotive & machine work |
| High strength | Grade 8 / class 10.9 | Suspension, engine, structural |
| Corrosion resistance | Stainless A2 / A4 | Marine, outdoor, food service |
| Extreme conditions | Specialty alloys | High temperature, chemical exposure |
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Grade 8 / class 10.9 bolt assortment
A graded, sorted kit of high-strength bolts, nuts and washers means you’ll have the right marked fastener on hand instead of reaching for an unmarked one ‘just this once’.
Stainless, zinc and galvanized
Letters and coatings tell you about corrosion resistance, not raw strength.
| Finish | Look for | Best for |
|---|---|---|
| Stainless steel | A2, A4, 304, 316 | Marine, food, chemical (corrosion) |
| Zinc plated | Bright or yellow coating | Indoor and light outdoor use |
| Hot-dip galvanized | “HDG”, thick dull-grey coat | Structural, outdoor construction |
Stainless steel (A2/A4) fastener assortment
A2 (304) for general outdoor work, A4 (316) for marine and salt exposure. A sorted box covers the common sizes so a rusty bolt never holds up the repair.
Anti-seize compound
A small brush-top tin pays for itself the first time a stainless or aluminium fastener comes apart cleanly instead of snapping off in the hole.
Read a head in three steps
Same routine whether it’s off a lawnmower or an engine block.
- Locate the marks — on top of the head, raised or stamped. Good light and a wipe of the surface help.
- Identify the system — radial lines mean SAE; a number like 8.8 means metric; letters like A2 or HDG describe the material or finish.
- Cross-reference and verify — match the marking to the tables above, and for any critical joint confirm against the manufacturer’s spec.
Frequently asked questions
What does a bolt head with no markings mean?
On an inch bolt, no radial lines means Grade 2 — low-carbon steel that’s fine for light-duty, non-structural work but not for anything that carries a real load. Unmarked imports can be lower still, so don’t trust a blank head on a critical joint.
How many lines is a Grade 8 bolt?
Six radial lines on the head. Grade 5 has three lines, and Grade 2 has none. The more lines, the stronger the bolt.
What does 10.9 stamped on a bolt mean?
It’s metric class 10.9: about 1040 MPa (150,000 psi) tensile strength, yielding at roughly 90% of that. In strength it’s the close cousin of an SAE Grade 8 bolt.
Is stainless steel stronger than Grade 8?
Usually no. Common A2 (304) and A4 (316) stainless resist corrosion but are weaker than Grade 8 or class 10.9. Choose stainless for wet or corrosive environments, not when you need maximum clamping force.
Can I mix metric and SAE fasteners?
No. Never thread a metric bolt into an inch nut or vice versa — they may start but won’t seat properly and can strip out. Match the system, the grade and the thread pitch.
Keep going
Bottom line: the head tells you the strength — count the radial lines on an SAE bolt, read the stamped number on a metric one, and treat letters as a finish, not a grade. Match or exceed the spec, choose the coating for the environment, and never step down on a joint that carries a load.