Understanding the jump from standard hardware-store bolts to medium-strength structural fasteners.
Proof strength: 33,000 psi (≤3/4") / 33,000 psi
Material: Low/medium carbon steel, no heat treatment
Head marking: None (unmarked)
Use: Light-duty, non-structural, furniture, fixtures
Proof strength: 85,000 psi (≤1")
Material: Medium carbon steel, quenched & tempered
Head marking: 3 radial lines
Use: Automotive, machinery, general structural
| Size | Grade 2 | Grade 5 | Difference |
|---|---|---|---|
| 1/4" | 5.5 ft·lbs | 8.4 ft·lbs | +53% |
| 5/16" | 11.3 ft·lbs | 17.4 ft·lbs | +54% |
| 3/8" | 20.0 ft·lbs | 30.9 ft·lbs | +54% |
| 7/16" | 32.0 ft·lbs | 49.4 ft·lbs | +54% |
| 1/2" | 48.8 ft·lbs | 75.4 ft·lbs | +55% |
| 9/16" | 70.4 ft·lbs | 108.8 ft·lbs | +55% |
| 5/8" | 97.1 ft·lbs | 150.1 ft·lbs | +55% |
| 3/4" | 172.2 ft·lbs | 266.2 ft·lbs | +55% |
| 7/8" | 166.8 ft·lbs | 429.5 ft·lbs | +157% |
| 1" | 250.0 ft·lbs | 643.9 ft·lbs | +158% |
Grade 2 is the baseline — unmarked, unhardened, and inexpensive. It's the bolt you find at the hardware store for hanging shelves, assembling furniture, or clamping non-load-bearing brackets. There's nothing wrong with Grade 2 in its proper role, but it has less than half the proof strength of Grade 5.
Grade 5 is the workhorse of industrial fastening. The quench-and-temper heat treatment more than doubles the proof strength, making it suitable for automotive engines, agricultural equipment, and structural connections. If the application sees any meaningful load, Grade 5 is the minimum.
The critical distinction: Grade 2 bolts will stretch and deform at loads where Grade 5 is still well within its elastic range. In any application with vibration, dynamic loading, or safety implications, Grade 5 is the starting point — not Grade 2.