Titanium Imperial (Inch) vs. Metric Bolts
1. Measurement Systems
| Feature | Imperial Bolts | Metric Bolts |
|---|
| Standard | ANSI/ASME B18.2.1 (USA/UK) | ISO 898-1 (International) |
| Thread Pitch | Threads per inch (TPI) | Distance between threads (mm) |
| Size Notation | Fractional inches (e.g., 1/4″) | Millimeters (e.g., M6) |
2. Thread Specifications
| Parameter | Imperial | Metric |
|---|
| Diameter | #4 to 1″ (e.g., 1/4-20) | M3 to M100 (e.g., M6-1.0) |
| Pitch Example | 1/4-20 = 20 threads per inch | M6-1.0 = 1.0 mm between threads |
| Thread Angle | 60° Unified (UNC/UNF) | 60° Metric (coarse/fine) |
3. Head Markings
| Type | Imperial | Metric |
|---|
| Grade Marks | Radial lines (e.g., 3 lines = Grade 5) | Numbers (e.g., 8.8 = 800 MPa tensile) |
| Common Grades | Grade 2, 5, 8 | 4.8, 8.8, 10.9, 12.9 |
4. Wrench Sizes
| Bolt Size | Imperial Wrench (in) | Metric Wrench (mm) |
|---|
| Small | 1/4″ → 7/16″ wrench | M6 → 10 mm wrench |
| Medium | 1/2″ → 3/4″ wrench | M12 → 18 mm wrench |
| Large | 1″ → 1-1/2″ wrench | M24 → 36 mm wrench |
5. Strength Comparison
| Grade | Tensile Strength (MPa) | Equivalent Metric Grade |
|---|
| Grade 2 | ~330 MPa | ~4.8 |
| Grade 5 | ~830 MPa | ~8.8 |
| Grade 8 | ~1030 MPa | ~10.9 |
6. Regional Usage
| Region | Primary Standard |
|---|
| USA, UK | Imperial (inch) |
| Europe, Asia, etc. | Metric |
| Automotive | Mixed (legacy Imperial in US) |
7. Key Identification Tips
- Imperial bolts:
- Measured in fractions (e.g., 3/8″) or numbers (#8, #10)
- Thread pitch = count threads per inch (e.g., 1/4-20 = 20 TPI)
- Metric bolts:
- “M” prefix + diameter (e.g., M8)
- Pitch follows (e.g., M8-1.25 = 1.25 mm pitch)
8. Compatibility Issues
- Never mix systems: Imperial and metric threads are incompatible.
- Tool confusion: Using an Imperial wrench on a metric bolt can round edges.
- DIY solutions: Adapters exist but reduce strength.
9. Conversion Cheat Sheet
| Imperial | Closest Metric | Notes |
|---|
| 1/4″ | M6 | Slightly smaller |
| 5/16″ | M8 | Close but not interchangeable |
| 3/8″ | M10 | Common automotive crossover |
| 1/2″ | M12 | Often confused in DIY projects |
10. When to Use Which?
- Choose Imperial:
- Repairing older American/British equipment
- Working with legacy machinery
- US automotive pre-1990s models
- Choose Metric:
- Modern vehicles (post-1990s)
- International projects
- Scientific/industrial equipment
Summary Table
| Factor | Imperial Bolts | Metric Bolts |
|---|
| Measurement | Inches (fractions) | Millimeters |
| Thread Pitch | TPI (threads per inch) | mm between threads |
| Strength Marks | Grade (e.g., Grade 5) | Class (e.g., 8.8) |
| Global Use | USA/UK dominant | Worldwide standard |
| Tool Compatibility | SAE wrenches | Metric wrenches |
