The two most commonly used high-strength bolt grades in North America. Here's how they compare in torque, strength, and application.
Proof strength: 85,000 psi (≤1") / 74,000 psi (>1")
Material: Medium carbon steel, quenched & tempered
Head marking: 3 radial lines
Use: Automotive, machinery, general structural
Proof strength: 120,000 psi
Material: Medium carbon alloy steel, quenched & tempered
Head marking: 6 radial lines
Use: Heavy equipment, high-stress, critical joints
| Size | Grade 5 | Grade 8 | Difference |
|---|---|---|---|
| 1/4" | 8.4 ft·lbs | 11.9 ft·lbs | +42% |
| 5/16" | 17.4 ft·lbs | 24.6 ft·lbs | +41% |
| 3/8" | 30.9 ft·lbs | 43.6 ft·lbs | +41% |
| 7/16" | 49.4 ft·lbs | 69.8 ft·lbs | +41% |
| 1/2" | 75.4 ft·lbs | 106.4 ft·lbs | +41% |
| 9/16" | 108.8 ft·lbs | 153.6 ft·lbs | +41% |
| 5/8" | 150.1 ft·lbs | 211.9 ft·lbs | +41% |
| 3/4" | 266.2 ft·lbs | 375.8 ft·lbs | +41% |
| 7/8" | 429.5 ft·lbs | 606.4 ft·lbs | +41% |
| 1" | 643.9 ft·lbs | 909.0 ft·lbs | +41% |
Choose Grade 5 for most general-purpose applications where cost matters. Grade 5 is the standard choice for automotive assembly, agricultural equipment, and non-critical structural connections. It offers a good balance of strength and affordability and is widely available at hardware stores.
Choose Grade 8 when the joint will see high static loads, vibration, or impact. Grade 8 is standard for heavy equipment (excavators, bulldozers), suspension and drivetrain components, and any bolted connection where failure could cause injury or significant damage. The ~40% higher proof strength justifies the price premium in critical applications.
Never mix grades in the same joint. If a design calls for Grade 8, using Grade 5 bolts could lead to joint failure. Going the other direction — using Grade 8 where Grade 5 is specified — is generally safe but wastes money and can complicate maintenance if replacement bolts aren't available in the field.
Grade 8 bolts are approximately 40% stronger than Grade 5, which means they require proportionally higher torque. Always verify the correct grade before tightening — under-torquing a Grade 8 bolt is as dangerous as over-torquing a Grade 5.