The Quick Rule for SAE Grades
SEO Meta Title: Bolt Grade Markings Guide - How to Read Head Markings | [Store Name]
SEO Meta Description: Learn to identify bolt grades by head markings. No marks = Grade 2, 3 lines = Grade 5, 6 lines = Grade 8. Metric, stainless & manufacturer marks explained.
Can't remember what those lines on a bolt head mean? This visual guide will help you identify bolt grades instantly by reading the head markings—no need to carry a reference chart.
The Quick Rule for SAE Grades
Count the radial lines and add 2:
| Head Marking | Grade | Tensile Strength |
|---|---|---|
| No marks (plain) | Grade 2 | 74,000 PSI |
| 3 radial lines | Grade 5 | 120,000 PSI |
| 6 radial lines | Grade 8 | 150,000 PSI |
| 6 lines + "9" | Grade 9 | 180,000 PSI |
Memory trick: 3 + 2 = 5, 6 + 2 = 8
SAE Grade Markings in Detail
Grade 2 — No Markings
Visual: Completely plain, unmarked hex head (may have manufacturer logo only)
What it means: Low carbon steel, no heat treatment, 74,000 PSI tensile strength. This is the "default" bolt—if there are no grade marks, assume Grade 2.
Where you'll find them: Hardware stores, home improvement centers, non-critical general fastening.
⚠️ Caution: A worn or corroded bolt may have markings you can't see. If it's critical, replace with a known grade.
Grade 5 — Three Radial Lines
Visual: Three lines radiating from center to edge of hex head, evenly spaced 120° apart. Lines are raised (embossed), not engraved.
What it means: Medium carbon steel, quenched and tempered, 120,000 PSI tensile strength. The standard automotive and industrial grade.
Pattern:
(Lines at 12 o'clock, 4 o'clock, and 8 o'clock positions)
Matching nuts: Also marked with 3 lines or 3 notches on the flats (not the top).
Grade 8 — Six Radial Lines
Visual: Six lines radiating from center, evenly spaced 60° apart, covering all six hex flats.
What it means: Medium carbon alloy steel, quenched and tempered, 150,000 PSI tensile strength. High-strength for critical applications.
Pattern:
(One line pointing toward each flat of the hex)
Matching nuts: Marked with 6 lines or 6 notches on the flats.
Common colors: Often yellow zinc plated (gold appearance), but color doesn't define grade—markings do.
Grade 9 — Six Lines Plus "9"
Visual: Six radial lines (like Grade 8) PLUS the numeral "9" or "G9" stamped on the head.
What it means: Alloy steel, quenched and tempered, 180,000 PSI tensile strength. Maximum SAE grade for extreme applications.
Pattern:
Key identifier: The "9" distinguishes it from Grade 8. Without the number, it's Grade 8.
Typical finish: Yellow zinc dichromate is common on Grade 9, but always verify by the "9" marking.
Metric Property Class Markings
Metric bolts are easier—the property class is stamped as a number directly on the head.
| Marking | Class | Approx. SAE Equivalent | Tensile Strength |
|---|---|---|---|
| 4.6 | Class 4.6 | Below Grade 2 | 400 MPa (58 ksi) |
| 4.8 | Class 4.8 | ~Grade 2 | 400 MPa (58 ksi) |
| 8.8 | Class 8.8 | ~Grade 5 | 800 MPa (116 ksi) |
| 9.8 | Class 9.8 | Between 5 & 8 | 900 MPa (131 ksi) |
| 10.9 | Class 10.9 | ~Grade 8 | 1,040 MPa (150 ksi) |
| 12.9 | Class 12.9 | > Grade 8 | 1,220 MPa (174 ksi) |
Reading Metric Markings
The first number Ă— 100 = approximate tensile strength in MPa
The second number Ă— the first Ă— 10 = approximate yield strength in MPa
Example: 10.9
- Tensile: 10 Ă— 100 = 1,000 MPa
- Yield: 10 Ă— 9 Ă— 10 = 900 MPa
Visual Location
Metric class numbers are stamped on top of the bolt head. May appear as:
- "8.8" (with decimal point)
- "88" (without point, older style)
- Sometimes preceded by manufacturer mark
Stainless Steel Markings
Stainless fasteners use alphanumeric codes indicating steel type and strength:
| Marking | Steel Type | Strength Class | Tensile Strength |
|---|---|---|---|
| A2-70 | 304 (18-8) | 70 | 700 MPa (102 ksi) |
| A2-80 | 304 (18-8) | 80 | 800 MPa (116 ksi) |
| A4-70 | 316 | 70 | 700 MPa (102 ksi) |
| A4-80 | 316 | 80 | 800 MPa (116 ksi) |
Breaking Down the Code
A2 or A4 = Stainless steel group
- A2 = Austenitic 304/18-8 (standard stainless)
- A4 = Austenitic 316 (marine grade, chloride resistant)
70 or 80 = Strength class
- 70 = 700 MPa tensile
- 80 = 800 MPa tensile (cold worked for extra strength)
Variations You May See
- A2 alone (without -70) = Assumes A2-70
- 18-8 = Common name for 304/A2 stainless
- 316 = Sometimes stamped instead of A4
- SS or SST = Generic "stainless" marking (verify actual grade)
Manufacturer Marks
Beyond grade markings, most bolts include a manufacturer's identification mark. These don't indicate strength—they identify who made it.
What Manufacturer Marks Look Like
- Single letter or two-letter code
- Logo or symbol
- Located on the head alongside grade markings
Why They Matter
1. Traceability: If a bolt fails, the mark identifies the manufacturer for investigation
2. Quality indicator: Reputable manufacturers mark their fasteners; unmarked bolts may be of unknown origin
3. Required by spec: ASTM, SAE, and ISO standards require manufacturer identification on many grades
Common Examples
- Triangle, circle, or diamond shapes with letters inside
- "D" inside a diamond = some manufacturers
- Various letter combinations (MW, FA, BS, etc.)
Note: Manufacturer marks vary widely. If you need to identify a specific manufacturer, contact us with a photo—we can often help identify the source.
Nut Markings
Nuts are marked differently than bolts—look for marks on the flats or chamfered faces, not the top.
SAE Nut Markings
| Grade | Marking Location | Mark Style |
|---|---|---|
| Grade 2 | None | No marking |
| Grade 5 | Top or flats | 3 notches or 3 dots |
| Grade 8 | Top or flats | 6 notches or 6 dots |
Metric Nut Markings
| Class | Marking |
|---|---|
| Class 8 | "8" on flat or top |
| Class 10 | "10" on flat or top |
| Class 12 | "12" on flat or top |
Clock position system: Some manufacturers use clock-position marking—a single notch at different positions to indicate grade.
When Markings Are Hard to Read
Corrosion and Wear
Old bolts may have markings worn away by:
- Rust and corrosion
- Repeated wrench contact
- Sandblasting or cleaning
- Paint over-coating
What to Do
1. Clean the head — Wire brush or solvent may reveal hidden marks
2. Check other bolts — If it's from a set, others may be more legible
3. Measure hardness — A Rockwell tester can estimate grade
4. When in doubt, replace — For any structural application, use new bolts with visible markings
The Safe Rule
If you can't identify the grade, treat it as Grade 2. Never assume an unmarked or illegibly marked bolt is high-strength.
Common Mistakes to Avoid
❌ Assuming color = grade
Yellow zinc bolts are often Grade 8, but color is just finish—not strength. Zinc-yellow can be applied to any grade. Always read the head markings.
❌ Mistaking manufacturer marks for grade marks
That logo isn't a strength indicator. Grade marks are radial lines (SAE) or numbers (metric), not symbols or letters.
❌ Counting the hex flats as "6 lines"
Grade 8 has 6 radial lines (raised marks on the head surface), not 6 hex flats (which every hex bolt has).
❌ Assuming imported bolts follow SAE standards
Metric bolts from overseas follow ISO standards, not SAE. An "8.8" marking means Class 8.8, not Grade 8.
❌ Using pliers or vice grips on bolt heads
This destroys markings and makes future identification impossible. Always use proper wrenches.
Quick Reference Card
Print this out for your toolbox:
Frequently Asked Questions
What if my bolt has lines AND numbers?
If it has 6 radial lines plus a "9", it's Grade 9. If it has numbers like "8.8", it's metric—ignore the "lines add 2" rule; just read the number.
Do stainless bolts have radial lines?
No. Stainless bolts use alphanumeric markings (A2-70, A4-80, etc.), not radial lines.
My bolt head says "SAE" — what grade is it?
"SAE" refers to the thread standard (SAE vs. metric), not the grade. Look for radial lines or numbers to determine grade.
Can I identify grade by measuring the bolt?
Not by dimensions. Grade affects strength, not size. However, a hardness tester can estimate grade based on Rockwell readings.
Why do some Grade 8 bolts have extra marks?
Manufacturers often add their logo alongside the 6 radial lines. The logo doesn't affect the grade—it just identifies the source.
Related Resources
- [Complete Bolt Grade Chart](/reference/bolt-grade-chart)
- [Grade 5 vs Grade 8: Which Do You Need?](/reference/grade-5-vs-8)
- [Metric vs SAE Grades Comparison](/reference/metric-vs-sae-grades)
- [Shop Bolts by Grade](/collections/bolts)
Still unsure about a bolt grade? Send us a clear photo of the head markings — [we'll identify it for you](/contact).