Bolt Grades Explained: Metric 8.8 vs SAE Grade 5

You are at the hardware store. You need a strong bolt. One bin says "Grade 8", another says "Class 10.9". Which one is stronger? Why does one have lines on the head and the other numbers?

Quick Answer (TL;DR)

• Metric: Uses Numbers (e.g., 8.8, 10.9). Higher = Stronger.
• US (SAE): Uses Lines (Radial dashes). More lines = Stronger.

Strength Comparison:

• SAE Grade 5 ≈ Metric 8.8 (Medium Strength, Automotive Standard)
• SAE Grade 8 ≈ Metric 10.9 (High Strength, Suspension/Engine)

Metric System (Property Classes)

Metric bolts have two numbers separated by a dot (e.g., 8.8).

1. The First Number (Ultimate Strength)

• Multiply by 100 to get Tensile Strength in Megapascals (MPa).
• Example (8.8): 8 × 100 = 800 MPa (breaking point).

2. The Second Number (Yield Strength)

• This is the percentage of the ultimate strength where the bolt permanently stretches.
• Example (8.8): .8 means 80%.
• Yield Strength = 80% of 800 MPa = 640 MPa.

Common Classes:

• Class 4.6: Low strength "hardware store" steel.
• Class 8.8: Medium carbon steel (Standard engineering bolt).
• Class 10.9: Alloy steel (High strength).
• Class 12.9: Super high strength (Allen/Socket heads).

US System (SAE J429 Grades)

US bolts use "Radial Lines" on the head to indicate grade.

Rule:

Grade = Number of Lines + 2

1. Grade 2 (No Lines)

• Low strength, mild steel.
• Use for: Wood, generic household fixes.
• Strength: ~60,000 - 74,000 psi.

2. Grade 5 (3 Lines)

• 3 lines + 2 = Grade 5

  .
• Medium strength, carbon steel.
• Use for: Automotive bracketry, general machinery.
• Strength: 120,000 psi (~825 MPa). Roughly equals Metric 8.8.

3. Grade 8 (6 Lines)

• 6 lines + 2 = Grade 8

  .
• High strength alloy steel.
• Use for: Suspension, engine heads, high stress.
• Strength: 150,000 psi (~1035 MPa). Roughly equals Metric 10.9.

Critical Warning: Don't Mix Them!

Dimensions: A Metric bolt (M10) will almost fit a US hole (3/8"), but will ruin the threads. Strength: Never replace a high-strength bolt (Grade 8 / 10.9) with a low-strength one (Grade 2 / 4.6). It will shear off.

Stainless Steel (The Exception)

Stainless bolts (usually marked A2-70 or 304) are great for rust, but they are weaker than Grade 8/10.9 bolts.

• Do not use Stainless Steel on high-stress suspension parts unless explicitly rated for it. They tend to be brittle or soft compared to heat-treated alloy steel.

Conclusion

• Automotive Work? Stick to Grade 5 / Class 8.8 minimum.
• Heavy Load? Upgrade to Grade 8 / Class 10.9.
• Markings: Look for Lines (US) or Numbers (Metric).
• Never downgrade a bolt's grade during repair.

Last Updated: February 16, 2026
Reading Time: 6 minutes