Essential thread types for high performance fasteners
Understanding thread types is critical for selecting fasteners that meet performance, safety, and compatibility requirements. This guide outlines the most common thread systems used in bolts, nuts, and studs—including Metric, Unified, UNJ, UNR, and Whitworth forms.
For over 50 years, Hague Fasteners has manufactured precision threaded fasteners in steels, stainless steels, and superalloys. Therefore, we know how thread selection affects strength, fatigue life, seal integrity, and interchangeability.
How to Identify a Thread
- System: Metric ISO, Unified, Whitworth, Pipe, Trapezoidal, Buttress
- Diameter and Pitch: Metric uses mm pitch; Unified/Whitworth use TPI (threads per inch)
- Series: Coarse, Fine, Extra Fine, Special
- Class of Fit: Metric (6g, 6H); Unified (1A/1B to 3A/3B)
- Form Geometry: ISO/Unified 60°, Whitworth 55°, UNJ/UNR radiused root, ACME 29°, Trapezoidal 30°
- Hand/Starts: Right or left-hand, single or multi-start
Metric & Metric Fine Threads (ISO, 60°)
Metric threads use pitch in millimetres. Coarse “M” is standard. Fine “MF” improves preload control and engagement in thin sections.
| Size | Coarse Pitch (M) | Common Fine Pitches (MF) | Angle | Notes |
|---|---|---|---|---|
| M6 | 1.0 | 0.75 | 60° | General purpose |
| M8 | 1.25 | 1.0 | 60° | Machinery, structures |
| M10 | 1.5 | 1.25, 1.0 | 60° | Better clamp control |
| M12 | 1.75 | 1.5, 1.25 | 60° | Thin sections |
| M16 | 2.0 | 1.5, 1.0 | 60° | Vibration resistance |
| M20 | 2.5 | 2.0, 1.5 | 60° | Flanged joints |
| M24 | 3.0 | 2.0, 2.5 | 60° | Pressure boundary |
| M30 | 3.5 | 2.0, 2.5, 3.0 | 60° | Large studs |
FAQs on Metric Threads
- What’s the difference? Metric Fine has smaller pitch for better preload control.
- When to use MF? Thin walls, short engagement, vibration resistance.
- Same angle? Yes—both use 60° flank angle.
Unified Threads UNC & UNF (ASME B1.1, 60°)
| Size (inch) | UNC (TPI) | UNF (TPI) | Angle | Notes |
|---|---|---|---|---|
| 1/4 | 20 | 28 | 60° | General fixtures |
| 3/8 | 16 | 24 | 60° | Preload control |
| 1/2 | 13 | 20 | 60° | Structural fasteners |
| 5/8 | 11 | 18 | 60° | Studs, couplings |
| 3/4 | 10 | 16 | 60° | Pressure equipment |
| 1 | 8 | 12 | 60° | Heavy joints |
FAQs on UNC & UNF Threads
- What’s the difference? UNC is coarse, easier to assemble. UNF is fine, better clamp control.
- Which is stronger? Depends on application—UNC for depth, UNF for engagement.
- Same angle? Yes—60° flank angle.
UNJ & UNR Threads (ASME, 60°)
UNJ threads feature a controlled root radius for improved fatigue strength. UNJC is coarse pitch, UNJF is fine pitch. UNR is a rounded-root version of UNC/UNF.
FAQs on UNJ & UNR Threads
- UNJ vs UNJF? UNJF is fine pitch within the UNJ family.
- UNJC vs UNJF? UNJC is coarse pitch, same root radius.
- What is UNR? Easier to roll, less controlled than UNJ but fatigue resistant.
- Where used? Aerospace, defence, rotating equipment.
Whitworth Threads BSW & BSF (BS 84, 55°)
Whitworth threads use a 55° flank angle with rounded crests and roots. BSW is coarse; BSF is fine.
FAQs on BSW & BSF Threads
- BSW vs BSF? BSF has finer pitch for thin sections.
- Interchangeable? No—Whitworth differs from Metric/Unified.
- Where found? Legacy British equipment, restoration, heritage plant.
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