Understanding thread standards is essential for selecting the right fasteners. Metric and imperial systems are not interchangeable, and using the wrong thread can cause catastrophic failure.
Metric Thread System (ISO)
The ISO metric thread is used worldwide (except USA). Designated as M followed by diameter and pitch:
- M6 x 1.0: 6mm diameter, 1.0mm pitch (coarse)
- M6 x 0.75: 6mm diameter, 0.75mm pitch (fine)
- M8 x 1.25: 8mm diameter, 1.25mm pitch (coarse)
- M10 x 1.5: 10mm diameter, 1.5mm pitch (coarse)
- M12 x 1.75: 12mm diameter, 1.75mm pitch (coarse)
Coarse vs Fine Threads
Coarse threads are standard for most applications. They resist cross-threading, assemble faster, and handle higher loads in soft materials.
Fine threads offer greater tensile strength, better vibration resistance, and finer adjustment. Used in aerospace, precision instruments, and high-stress applications.
Imperial Thread System (UNC/UNF)
Used primarily in the United States. Designated by diameter and threads per inch (TPI):
- 1/4-20 UNC: 1/4 inch diameter, 20 threads per inch (coarse)
- 1/4-28 UNF: 1/4 inch diameter, 28 threads per inch (fine)
- 3/8-16 UNC: 3/8 inch diameter, 16 TPI
- 1/2-13 UNC: 1/2 inch diameter, 13 TPI
Key Differences
- Thread angle: Both use 60 degrees, but pitch diameter and form differ
- Not interchangeable: A metric bolt will NOT properly engage an imperial nut
- Closest sizes: M6 is close to 1/4 inch, M8 close to 5/16, M10 close to 3/8 – but NOT compatible
Thread Classes and Tolerance
Metric (ISO 965)
- 6g: Standard external thread (bolt) tolerance
- 6H: Standard internal thread (nut) tolerance
- 4h6h: Tighter tolerance for precision applications
Imperial (ASME B1.1)
- Class 1A/1B: Loose fit, easy assembly
- Class 2A/2B: Standard fit, most common
- Class 3A/3B: Tight fit, precision applications
Special Thread Forms
- BSW/BSF: British Standard Whitworth/Fine (55 degree angle)
- BSP: British Standard Pipe threads for fittings
- NPT: National Pipe Taper for US pipe fittings
- ACME: Trapezoidal threads for lead screws
Conversion Tips
Never force a metric bolt into an imperial hole or vice versa. If you must convert, use adapter nuts or re-tap the hole to the correct thread standard.
Chaoshuo Trading – We supply both metric and imperial fasteners in all standard sizes. Custom thread forms available on request.