Thread Types Guide
Choosing the correct thread type is critical for fastener performance. This guide covers all major thread standards used in brass fasteners with pitch charts and application notes.
Metric Threads (M)
Metric threads are the global standard, designated by 'M' followed by the nominal diameter. Available in coarse and fine pitch variants. Used across European, Asian and most international applications.
| Designation | Diameter (mm) | Coarse Pitch (mm) | Fine Pitch (mm) |
|---|---|---|---|
| M3 | 3 | 0.5 | 0.35 |
| M4 | 4 | 0.7 | 0.5 |
| M5 | 5 | 0.8 | 0.5 |
| M6 | 6 | 1.0 | 0.75 |
| M8 | 8 | 1.25 | 1.0 |
| M10 | 10 | 1.5 | 1.25 |
| M12 | 12 | 1.75 | 1.25 |
| M16 | 16 | 2.0 | 1.5 |
| M20 | 20 | 2.5 | 1.5 |
UNC — Unified National Coarse
UNC threads are the standard inch-based thread in North America. Coarser pitch makes them faster to assemble and more tolerant of contamination. Widely used in USA and Canada.
| Size | Threads Per Inch (TPI) | Pitch (inch) | Standard |
|---|---|---|---|
| #4-40 | 40 | 0.025 | ASME B1.1 |
| #6-32 | 32 | 0.031 | ASME B1.1 |
| #8-32 | 32 | 0.031 | ASME B1.1 |
| #10-24 | 24 | 0.042 | ASME B1.1 |
| 1/4-20 | 20 | 0.050 | ASME B1.1 |
| 5/16-18 | 18 | 0.056 | ASME B1.1 |
| 3/8-16 | 16 | 0.063 | ASME B1.1 |
| 1/2-13 | 13 | 0.077 | ASME B1.1 |
UNF — Unified National Fine
UNF threads have a finer pitch than UNC, providing greater tensile strength and better resistance to loosening under vibration. Used in automotive and precision applications.
| Size | Threads Per Inch (TPI) | Pitch (inch) | Standard |
|---|---|---|---|
| #4-48 | 48 | 0.021 | ASME B1.1 |
| #6-40 | 40 | 0.025 | ASME B1.1 |
| #8-36 | 36 | 0.028 | ASME B1.1 |
| #10-32 | 32 | 0.031 | ASME B1.1 |
| 1/4-28 | 28 | 0.036 | ASME B1.1 |
| 5/16-24 | 24 | 0.042 | ASME B1.1 |
| 3/8-24 | 24 | 0.042 | ASME B1.1 |
| 1/2-20 | 20 | 0.050 | ASME B1.1 |
BSP / BSPT — British Standard Pipe Threads
BSP (parallel) and BSPT (tapered) threads are used in plumbing and hydraulic fittings. Common in UK, Australia, Asia and other markets using British standards.
| Size | TPI | Application | Standard |
|---|---|---|---|
| 1/8" | 28 | Plumbing fittings | BS 2779 |
| 1/4" | 19 | Hydraulic fittings | BS 2779 |
| 3/8" | 19 | Gas fittings | BS 2779 |
| 1/2" | 14 | Water pipe fittings | BS 2779 |
| 3/4" | 14 | Industrial fittings | BS 2779 |
| 1" | 11 | Large bore plumbing | BS 2779 |
Frequently Asked Questions
What is the difference between coarse and fine threads?
Coarse threads (UNC, metric coarse) assemble faster and are more tolerant of dirt. Fine threads (UNF, metric fine) provide higher tensile strength and better vibration resistance.
Which thread type is best for plumbing applications?
BSP threads are standard for plumbing in most markets. For USA plumbing, NPT threads are used. RKIC Group manufactures brass fittings in both BSP and NPT.
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