Fastening Know-How
Bolt Grades and Head Markings: SAE Grade 2, 5 and 8 Chart
How strong SAE Grade 2, 5 and 8 bolts are, how to read the marks on the head, and how structural, metric and stainless markings compare.
What a Bolt Grade Tells You
A bolt grade sets the minimum strength a fastener must reach. Inch fasteners used in automotive and related industries are graded under SAE J429, which covers sizes up to 1-1/2 inches. The three grades most buyers meet are Grade 2 (low strength), Grade 5 (medium strength) and Grade 8 (high strength). Always replace a graded fastener with one of the same or a higher grade.
SAE J429 Bolt Grade Chart
Minimum mechanical properties for the three common SAE J429 grades.
| Grade | Size range | Proof load (psi) | Yield strength, min (psi) | Tensile strength, min (psi) | Core hardness (Rockwell) | Material |
|---|---|---|---|---|---|---|
| Grade 2 | 1/4" to 3/4" | 55,000 | 57,000 | 74,000 | B80 to B100 | Low or medium carbon steel |
| Grade 2 | Over 3/4" to 1-1/2" | 33,000 | 36,000 | 60,000 | B70 to B100 | Low or medium carbon steel |
| Grade 5 | 1/4" to 1" | 85,000 | 92,000 | 120,000 | C25 to C34 | Medium carbon steel, quenched and tempered |
| Grade 5 | Over 1" to 1-1/2" | 74,000 | 81,000 | 105,000 | C19 to C30 | Medium carbon steel, quenched and tempered |
| Grade 8 | 1/4" to 1-1/2" | 120,000 | 130,000 | 150,000 | C33 to C39 | Medium carbon alloy steel, quenched and tempered |
Grade 2 values for 1/4" to 3/4" apply to bolts 6" and shorter and to studs of all lengths. Sources: Southwest Bolt, SAE J429 technical specifications (2020) and Reliable Fasteners, Reli-A-Pak SAE J429 data sheet, matched value by value (accessed 25 Sep 2026). This is a summary; check the current edition of SAE J429 before relying on it for design.
How to Read Bolt Head Markings
Inch bolts carry raised radial lines on the head. Count the lines and add two: three lines is Grade 5, six lines is Grade 8. A plain head usually means Grade 2, but not always, so check the other marks before you assume.
| Grade | Head marking | Material | Minimum tensile strength | Typical use |
|---|---|---|---|---|
| Grade 2 | No radial lines | Low or medium carbon steel | 74,000 psi (1/4" to 3/4") | Light, non-critical joints |
| Grade 5 | 3 radial lines, 120 degrees apart | Medium carbon steel, quenched and tempered | 120,000 psi (1/4" to 1") | General automotive and machinery |
| Grade 8 | 6 radial lines, 60 degrees apart | Medium carbon alloy steel, quenched and tempered | 150,000 psi (1/4" to 1-1/2") | Suspension, drivetrain, heavy equipment |
| Grade 8.2 | 6 radial lines plus an 8.2 stamp | Low carbon boron steel, quenched and tempered | 150,000 psi | Room-temperature alternative to Grade 8 |
| ASTM A307 | "307A" or "307B" stamp, no lines | Low carbon steel | 60,000 psi | Construction and general purpose |
Grade 8.2 matches Grade 8 tensile strength but is not the same part at elevated temperature. A head with no lines is not automatically Grade 2: look for a 307A stamp.
Proof Load, Yield and Tensile Strength
- Proof load is the tension a bolt must carry without any permanent stretch.
- Yield strength is the stress at which the bolt starts to stretch permanently.
- Tensile strength is the stress at which it breaks.
Design to proof load and yield, never to tensile strength: a bolt loaded past yield has already lost its clamp.
Metric Property Classes and Larger Sizes
Metric fasteners use property classes instead of grades. As a rough guide, class 8.8 is similar to Grade 5, class 10.9 to Grade 8, and class 12.9 exceeds Grade 8. These are comparisons, not equivalents; specify the class your drawing calls for.
SAE J429 stops at 1-1/2 inches. For larger diameters, ASTM A307 is the usual substitute for Grade 2, ASTM A449 for Grade 5 and ASTM A354 Grade BD for Grade 8. They are similar but not identical, so confirm before substituting.
Compare structural grades in A325 vs A490 vs Grade 8 vs B7, see dimensions in the bolt size chart, or request a quote for graded fasteners.
Sources: Fastener Mart, “Understanding Fastener Grades and Classes”; Southwest Bolt SAE J429 sheet; HexCapScrews.com SAE J429 reference (accessed 25 Sep 2026).
Structural Bolt Markings: ASTM F3125 (A325 and A490)
Heavy hex structural bolts for steel-to-steel connections are covered by ASTM F3125. In 2016 ASTM withdrew the separate A325 and A490 specifications and folded them into F3125 as grades, so the head still carries an “A325” or “A490” stamp next to the manufacturer mark.
- Type 1 is standard carbon or alloy steel.
- Type 3 is weathering steel and is marked with the grade name underlined.
- Galvanizing: F3125 does not allow hot-dip galvanizing of A490 bolts because of hydrogen embrittlement risk. A galvanized bolt stamped A490 should be rejected at receiving.
Metric Property Class Markings
Metric bolts carry a two-part number instead of lines. Multiply the first number by 100 for the nominal tensile strength in MPa; multiply the second by 10 for yield as a percentage of tensile.
| Head marking | Nominal tensile | Yield (share of tensile) | Nearest inch grade |
|---|---|---|---|
| 8.8 | 800 MPa | 80% (640 MPa) | Close to Grade 5 |
| 10.9 | 1,000 MPa | 90% (900 MPa) | Close to Grade 8 |
| 12.9 | 1,200 MPa | 90% (1,080 MPa) | Above Grade 8 |
Nearest-grade comparisons are a starting point for a substitution review, never a drop-in swap on a controlled print.
Stainless Steel Bolt Markings (A2 and A4)
Stainless fasteners follow ISO 3506 and use a letter code, not lines. A2 is the 304 family for general atmospheric use. A4 is the 316 family, where added molybdenum gives better resistance to salt and chlorides. The number after the dash is minimum tensile strength divided by 10, so A4-80 is a 316-family bolt with 800 MPa minimum tensile. Stainless runs softer than hardened carbon steel and can gall, so check the class before you set a torque.
Check the Manufacturer Mark and the Paperwork
The grade mark tells you the strength class. The manufacturer mark, the letter or symbol beside it, tells you who made the part. Under the Fastener Quality Act those insignia are registered, which is what makes a counterfeit traceable. A missing or blurred manufacturer mark is the first warning sign.
A stamp is only a claim. A mill test report tied to the lot proves the chemistry and mechanical results. At receiving, sample every incoming lot, confirm the lines, property class and manufacturer mark against the purchase order, keep strength classes in separate labelled bins, and never top up a bin from a different lot.
Bolt Grade and Marking FAQ
How do you tell the difference between Grade 5 and Grade 8 bolts?
Count the radial lines on the head. Grade 5 has three lines 120 degrees apart; Grade 8 has six lines 60 degrees apart. Grade 8 carries 150,000 psi minimum tensile strength against 120,000 psi for Grade 5 in sizes up to 1 inch.
What do the letters on a bolt head mean?
A letter or symbol on its own is usually the manufacturer identification mark. A spelled-out name such as A325 or A490 is a structural grade under ASTM F3125, and a code such as A2-70 or A4-80 is a stainless grade under ISO 3506.
What does 8.8 mean on a metric bolt?
It is the property class. 8 means a nominal tensile strength of 800 MPa, and .8 means yield is 80 percent of that, or 640 MPa.
Is a bolt with no markings Grade 2?
Usually, but check for other stamps first. ASTM A307 bolts carry a 307A or 307B stamp instead of lines, and unmarked hardware of unknown origin should not go into a controlled joint.
How can I spot a counterfeit or mismarked bolt?
Look for a clear, registered manufacturer mark next to the grade mark, check that the finish is allowed for the grade (a galvanized A490 is not), and ask for a mill test report tied to the lot.
What is stronger, Grade 8 or class 10.9?
They are close. Class 10.9 has a nominal tensile strength of 1,000 MPa (about 145,000 psi) and Grade 8 has a minimum of 150,000 psi. Treat them as comparable, not interchangeable, and follow the class or grade on the drawing.
Specifying a Safety-Critical Joint?
These charts are reference values drawn from published standards. Send us your drawing or specification and our team will review the fastener choice with you.