Bolt Grade Chart
| Grade / spec | Material | Size range (in) | Proof (ksi) | Yield (ksi) | Tensile (ksi) | Head marking |
|---|---|---|---|---|---|---|
| SAE J429 Grade 1 | Low or medium carbon steel | 1/4 - 1-1/2 | 33 | 36 | 60 | none |
| SAE J429 Grade 2 | Low or medium carbon steel | 1/4 - 3/4 | 55 | 57 | 74 | none |
| SAE J429 Grade 2 | Low or medium carbon steel | 7/8 - 1-1/2 | 33 | 36 | 60 | none |
| SAE J429 Grade 5 | Medium carbon steel, quenched and tempered | 1/4 - 1 | 85 | 92 | 120 | 3 radial lines |
| SAE J429 Grade 5 | Medium carbon steel, quenched and tempered | 1-1/8 - 1-1/2 | 74 | 81 | 105 | 3 radial lines |
| SAE J429 Grade 8 | Medium carbon alloy steel, quenched and tempered | 1/4 - 1-1/2 | 120 | 130 | 150 | 6 radial lines |
| ASTM A307 Grade A | Low carbon steel | 1/4 - 4 | — | — | 60 | none |
| ASTM F3125 A325 Type 1 | Medium carbon steel, quenched and tempered | 1/2 - 1-1/2 | 85 | 92 | 120 | 3 radial lines (120° apart) |
| ASTM F3125 A490 Type 1 | Alloy steel, quenched and tempered | 1/2 - 1-1/2 | 120 | 130 | 150 min - 173 max | 6 radial lines (60° apart) |
| ASTM A449 | Medium carbon steel, quenched and tempered | 1/4 - 1 | 85 | 92 | 120 | 3 radial lines |
| ASTM A193 B7 | Alloy steel, quenched and tempered | up to 2-1/2 | — | 105 | 125 | B7 |
| ASTM A354 Grade BD | Alloy steel, quenched and tempered | 1/4 - 4 | — | 130 | 150 min - 173 max | 6 radial lines (60° apart) |
The markings on a bolt head are a promise in code: three radial lines mean the fastener met SAE Grade 5 requirements, six lines mean Grade 8, and a blank head means the softest Grade 1 or 2 stock. The grade is the tensile story in shorthand - Grade 2 holds about 74,000 pounds per square inch, Grade 5 holds 120,000, Grade 8 holds 150,000. The chart below carries the full strength record for the everyday SAE J429 ladder plus the ASTM cousins that share the shelves (A307 for cheap structural work, the A325/A490 structural pair, A449, the A193 B7 pressure-vessel staple and A354 BD), with proof, yield and tensile values side by side so a substitution decision takes one glance instead of a standards-library afternoon.
Two patterns in the table do most of the thinking. First, the grade ladder climbs almost perfectly linearly: from Grade 2 to 5 to 8, tensile strength steps 74 to 120 to 150 ksi while proof and yield climb in lockstep - the heat treatment, not the steel alone, buys each step. Second, bigger is weaker per size class: Grade 2 above 3/4-inch diameter drops back to 60 ksi tensile, and Grade 5 above 1-inch drops to 105. That is not a typo - it is the quench. Hardness from heat treatment penetrates only so deep, so a large-diameter bolt cannot carry the same strength rating as a small one of the same grade. Anyone substituting a bigger bolt for a smaller one on strength grounds needs this row of the table before, not after, the swap.
How to use
- Identify the bolt first: count the radial lines on the head (none, 3 or 6) or read the stamp (B7, A325, BD), then find that marking in the chart - the marking and the grade are a one-to-one pair in the SAE system.
- Read the strength row against the job: proof load is the working ceiling the bolt springs back from, yield is where permanent stretch begins, and tensile is the breaking point - size the joint to proof, never to tensile, or the bolt creeps loose in service.
- Check the diameter band before trusting a number: the table splits Grade 2 and Grade 5 into two rows because the rating changes with diameter - a 1-1/4 inch Grade 5 is 105 ksi tensile, not the 120 its small brothers carry.
Frequently asked questions
When should I use Grade 5 instead of Grade 8, or the other way round?
Grade 8 is stronger in every static measure - 150 ksi tensile and 130 ksi yield against 120 and 92 - so for a straight strength-limited joint, Grade 8 wins. The reason Grade 5 survives in general service is ductility and the reality of shock loads: the softer Grade 5 stretches visibly before it fails, which is a warning you can see during inspection, while a Grade 8 loaded past its yield heads to fracture with less announcement. Clamping joints that get taken apart often, sawmill and plow applications, and any joint where a technician will check tightness by feel tend to spec Grade 5; high-strength static joints, suspension components and places where the bolt must simply never yield spec Grade 8. One rule survives every shop argument: never mix grades in the same joint - the soft bolt unloads the hard one and the clamp load the designer counted on quietly disappears.
What do metric property classes like 8.8 and 10.9 mean, and what do they match in SAE grades?
The metric number is the strength written out, not a rank. Multiply the first digit by 100 to get tensile in megapascals: 8.8 is roughly 800 MPa tensile. Multiply that by the second digit to get yield: 0.8 times 800 is about 640 MPa yield. So 10.9 means roughly 1,000 MPa tensile with 90 percent of that as yield, and 12.9 is 1,200 with the same ratio - the strongest common class. As rough walking comparisons, an 8.8 sits near SAE Grade 5 territory and a 10.9 near Grade 8, with 12.9 above both - but near is not interchangeable: the two systems measure on different scales (MPa versus psi), the proof-load definitions differ in the fine print, and grade markings are not checked against each other by any standard. Treat the comparison as a sizing hint, verify the joint, and never let a metric bolt stand in for an SAE one on paper strength alone.
Why does the same grade have a lower rating in bigger diameters?
Heat treatment depth, written in steel. Grades 5 and 8 get their strength from quenching and tempering, and a quenched surface hardens faster than the core of a thick section - above about one inch of diameter, the center of the bar cannot cool fast enough to reach the same structure. The standards committee answered with two rating bands: Grade 5 up to 1 inch carries 120 ksi tensile, from 1-1/8 up it is rated 105; Grade 2 drops from 74 to 60 ksi above 3/4 inch. The practical trap is upsizing: swapping a stripped 1/2-inch Grade 5 for a 1-1/4-inch bolt feels like a pure upgrade, but the big bolt carries less strength per unit area - and if the hole pattern was designed for clamp load at the smaller size, the substitution can end up weaker in practice. Check the diameter row before assuming bigger means stronger.
What is ASTM A307, and how is it different from SAE Grade 2?
A307 is the economy structural grade: low-carbon steel, a 60 ksi minimum tensile requirement and essentially no hardness or yield engineering - which is why the chart shows dashes in its proof and yield columns. Grade 2 in small diameters (1/4 to 3/4 inch) must hit 74 ksi tensile and 57 ksi yield, so on paper it outruns A307. In large diameters the two nearly merge: Grade 2 above 3/4 inch is rated at the same 60 ksi tensile as A307. The real difference is intent and paperwork: A307 exists for construction - anchor bolts, flanges, timber and structural attachments where strength demands are modest and price matters - while Grade 2 is the general hardware-store machine bolt. They are not certified substitutes for each other because the certification a job site asks for names one specification, and inspectors check the head marking against the paperwork, not the strength table.