How to Read Bolt Head Markings: Grade Stamps Explained
You pulled a bolt out of your lawnmower deck, dishwasher motor mount, or car suspension bracket and now you need to replace it. The hardware store bin has three different looking bolts in the same size — and you're not sure which one matches. The answer is stamped right on the head. You just need to know how to read it.
This guide covers every common marking system: SAE grades (the radial line count), metric property classes (the two-digit number), and what to do when the head has nothing on it at all.
SAE Bolt Grades: Count the Radial Lines
Standard hex bolts sold in the US follow SAE J429 specifications. The grade is indicated by radial lines stamped on the top face of the hex head — the flat face you see when looking straight down at the bolt. More lines means higher grade, higher strength.
The lines can be faint — especially on older or weathered bolts. Use a flashlight held at a low angle to see them clearly. Run your fingernail across the head: grade markings are stamped in (raised on some), not painted on.
SAE Grade Strength Specifications
| Grade | Lines | Proof Load | Tensile | Typical Use |
|---|---|---|---|---|
| Grade 2 | None | 55–57 ksi | 74 ksi | Light fixtures, cabinetry |
| Grade 5 | 3 | 85 ksi | 120 ksi | Engines, appliances, deck ledgers |
| Grade 8 | 6 | 120 ksi | 150 ksi | Suspension, drivetrain, structural |
Metric Property Classes: Reading the Numbers
Metric hex bolts follow ISO 898-1 and stamp a two-number property class directly on the head — for example 8.8, 10.9, or 12.9. There are no radial lines. Once you know the decoding formula, the numbers give you the strength directly.
Decoding the Property Class Number
The two numbers follow a simple formula:
- First number × 100 = nominal tensile strength in MPa. Class 10.9 → 10 × 100 = 1,000 MPa minimum.
- Both numbers × 10 = nominal yield strength in MPa. Class 10.9 → 10 × 9 × 10 = 900 MPa minimum.
Class 12.9 (socket cap screws are often 12.9) is actually stronger than Grade 8. If you're sourcing a socket head cap screw to replace a Grade 8 hex bolt, a 12.9 socket cap exceeds the Grade 8 spec — but confirm it's the right size and thread pitch first.
SAE vs Metric Grade Cross-Reference
| SAE Grade | Head Mark | Metric Equiv. | Tensile (approx) |
|---|---|---|---|
| Grade 2 | None | Class 4.6 | 60–74 ksi / 400–510 MPa |
| Grade 5 | 3 lines | Class 8.8 | 120 ksi / 830 MPa |
| Grade 8 | 6 lines | Class 10.9 | 150 ksi / 1,040 MPa |
| — (no equiv.) | — | Class 12.9 | 177 ksi / 1,220 MPa |
Other Markings You'll See
Manufacturer Stamps
Many bolts also stamp a manufacturer identifier — letters or a logo — on the head alongside or instead of grade lines. An "F" + numbers (like "F574" or "F835") indicates an ASTM specification. Common ASTM specs:
- ASTM A307 — Low-strength all-purpose (comparable to Grade 2). Head often stamped "A307."
- ASTM A325 — High-strength structural bolt (~120 ksi, similar to Grade 5). Common in steel construction.
- ASTM A490 — Extra high-strength structural (~150 ksi, similar to Grade 8). Heavy structural steel.
Stainless Steel Markings
Stainless bolts carry different stampings. Look for "A2" or "A4" (the ISO code) or "18-8" (referring to 18% chromium, 8% nickel composition). These aren't SAE grades — they indicate alloy, not strength class specifically. A2-70 is the most common combination you'll see on stainless fasteners: alloy A2 at property class 70 (700 MPa tensile, roughly similar to Grade 5).
Unheaded Bolts and Studs
Fully-threaded studs and carriage bolts don't have a hex head to stamp, so grade verification requires the original spec sheet or a test. When in doubt on a stud replacement, go to your vehicle's service manual or appliance's parts list — grade matters more on studs than on bolts because they can't be visually verified in service.
What to Do If the Head Has No Markings
An unmarked hex bolt is almost certainly Grade 2. The decision tree:
- If the original application used unmarked bolts (cabinetry hardware, light fixtures, non-structural) → Grade 2 replacement is fine.
- If you're unsure what grade was there originally → Replace with Grade 5. It covers most appliance and light structural applications without over-engineering.
- If the joint is structural, load-bearing, or vibration-exposed → Get the spec from the service manual. Never guess on suspension, blade mounting, or trailer hitches.
Identify Your Fastener Now
Once you know the grade, use our free identifier tool to confirm thread pitch, diameter, and length from your measurements.
Open Identifier →Stainless vs. Zinc-Plated vs. Uncoated
Grade markings tell you strength, not corrosion resistance. The finish or coating is a separate indicator:
| Finish | Appearance | Corrosion Resistance | Best For |
|---|---|---|---|
| Plain / Bare | Dull silver-grey | Minimal | Indoor, dry applications only |
| Zinc plated (yellow/silver) | Shiny silver or golden tint | Moderate (200–300 hr salt spray) | General hardware, light outdoor |
| Hot-dip galvanized | Matte grey, rough texture | High (>1,000 hr salt spray) | PT lumber, outdoor structural |
| Stainless A2/A4 | Polished or brushed silver | Very high (A4 for marine) | Outdoor, marine, food service |
One practical note: hot-dip galvanized coatings add thickness (~5 mils) that slightly changes thread fit. If the original bolt screwed in with normal torque, a galvanized replacement may be a slightly tighter fit on the nut — this is normal, not a wrong-thread situation.
Appliance and Auto Repair: Matching Grade to Application
In real repair scenarios, here's what grade to reach for:
| Application | Minimum Grade | Notes |
|---|---|---|
| Appliance cover panel | Grade 2 | Low load, vibration minimal |
| Washer motor mount | Grade 5 | Vibration load — don't go lower |
| Lawnmower blade | Grade 8 | High centrifugal force — spec exactly |
| Car exhaust flange | Grade 5 (or OEM spec) | High heat — use anti-seize compound |
| Wheel lug bolts | OEM only | Never substitute — use manufacturer part |
| Suspension arm bolts | Grade 8 / Class 10.9 | Safety-critical — always match OEM grade |
Recommended Tool
CLBDRESS 50-Piece Thread Checker Kit
Once you've identified the grade from the head marking, you still need the thread pitch to get an exact replacement. This 50-piece kit covers metric and SAE thread gauges from M2 to M18 and 4-40 to 1/2-20 — it's the fastest way to confirm you're buying the right bolt at the hardware store.
View Thread Checker Kits on Amazon →Frequently Asked Questions
What do the lines on a bolt head mean?
Radial lines stamped on a SAE hex bolt head indicate its grade. No lines = Grade 2 (lowest common grade). Three lines = Grade 5 (medium strength, 120 ksi tensile). Six lines = Grade 8 (high strength, 150 ksi tensile). More lines always means stronger.
How do you read metric bolt head markings?
Metric bolts stamp a property class number on the hex head face, such as 8.8, 10.9, or 12.9 — no radial lines. The first number × 100 gives the approximate tensile strength in MPa. So Class 10.9 = ~1,000 MPa (≈151 ksi), roughly equivalent to SAE Grade 8.
What does a bolt with no markings mean?
An unmarked hex bolt is typically Grade 2 — low-carbon steel with ~74 ksi tensile strength. Don't use it in structural, vibrating, or high-load applications. If the original bolt had markings and you can't find matching grade hardware, go to a fastener specialty store rather than using unmarked replacements.
Can you substitute Grade 5 for Grade 8?
No — not in any application where Grade 8 was specified. Grade 5 has a 41% lower proof load (85 ksi vs. 120 ksi). Blade bolts, suspension bolts, trailer hitch bolts, and similar safety-critical fasteners must be replaced with the original grade or higher.
What is the metric equivalent of Grade 8?
Class 10.9 is the metric equivalent of SAE Grade 8. Both have tensile strengths around 150 ksi (1,040 MPa). Class 8.8 approximates Grade 5. These property classes are stamped on the bolt head as two digits with a decimal point.