Thread Types and Forms Guide for Fasteners
This Technical Bulletin explains the major thread types used on bolts, nuts, and studs. It covers Metric and Metric Fine, Unified series UNC, UNF, UNEF, UNS, UNJC, UNJF, UNR, Whitworth BSW and BSF, plus pipe, trapezoidal, buttress, and specialist forms. Use it to identify a thread system, select a class of fit, and avoid specification errors on safety critical fasteners.
For over 50 years, Hague Fasteners has manufactured precision threaded fasteners in steels, stainless steels, and superalloys. Therefore, we understand how thread selection influences strength, fatigue life, seal performance, and interchangeability. This guide consolidates the essentials so engineers can choose with confidence.
How to Identify a Thread
- System: Metric ISO, Unified, Whitworth, Pipe, Trapezoidal, Buttress.
- Diameter and pitch: Metric uses pitch in mm. Unified/Whitworth use TPI (threads per inch).
- Series: Coarse, Fine, Extra Fine, Special.
- Class of fit: Metric uses 6g, 6H, etc. Unified uses 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 the default. Fine “MF” improves preload control and engagement in thin sections. Flank angle is 60 degrees.
| 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 with MF |
| 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 |
Frequently Asked Questions on Metric & Metric Fine Threads
What is the difference between Metric and Metric Fine?
Metric (M) threads are coarse pitch. Metric Fine (MF) has a smaller pitch, giving more engaged threads and better preload control.
When should Metric Fine be used?
On thin wall sections, short engagement lengths, or where vibration resistance or precise clamp is required.
Do they use the same angle?
Yes. Both use a 60° flank angle under ISO 261/965. Only the pitch differs.
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° | UNF for 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 |
Frequently Asked Questions on UNC & UNF Threads
What is the difference?
UNC has fewer TPI, easier to assemble. UNF has more TPI, better clamp control and vibration resistance.
Which is stronger?
Depends. UNC spreads load deeper. UNF has more engagement in short lengths. Selection depends on design.
Do they use the same angle?
Yes. Both UNC and UNF use a 60° flank angle.
UNJ & UNR Threads (ASME, 60°)
UNJ adds a controlled root radius to improve fatigue strength. UNJC is coarse pitch, UNJF is fine pitch. UNR is a rounded-root version of UNC/UNF.
| Size (inch) | UNJF (TPI) | UNJC (TPI) | UNR (TPI) | Angle | Notes |
|---|---|---|---|---|---|
| 1/4 | 28 | 28 | UNC 20 / UNF 28 | 60° | Fatigue-critical small studs |
| 3/8 | 24 | 24 | UNC 16 / UNF 24 | 60° | Engine bolts |
| 1/2 | 20 | 20 | UNC 13 / UNF 20 | 60° | Pressure studs |
| 3/4 | 16 | 16 | UNC 10 / UNF 16 | 60° | Rotating assemblies |
Frequently Asked Questions on UNJ & UNR Threads
What is the difference between UNJ and UNJF?
UNJ specifies a root radius. UNJF indicates a fine pitch within the UNJ family.
How is UNJC different from UNJF?
UNJC is coarse pitch, UNJF is fine pitch. Both have the same radius root form.
What are UNR threads?
Rounded-root versions of UNC/UNF, easier to roll. Less controlled than UNJ but still fatigue resistant.
Where are UNJ threads used?
Aerospace, defence, rotating equipment, turbine studs, and fatigue-critical bolts.
Whitworth Threads BSW & BSF (BS 84, 55°)
| Size (inch) | BSW (TPI) | BSF (TPI) | Angle | Notes |
|---|---|---|---|---|
| 1/4 | 20 | 26 | 55° | Legacy fixings |
| 3/8 | 16 | 20 | 55° | Restoration work |
| 1/2 | 12 | 16 | 55° | Older flanges |
| 3/4 | 10 | 12 | 55° | Heritage machinery |
| 1 | 8 | 10 | 55° | Pipework, heavy plant |
Frequently Asked Questions on BSW & BSF Threads
What is the difference?
BSW is coarse, BSF is fine. Both use 55° flank angle with rounded crests and roots.
When should BSF be used?
On thin sections or short engagement requiring more threads for load.
Are they interchangeable with Metric/Unified?
No. Whitworth 55° differs from Metric/Unified 60°. Adaptors are required.
Where are they still found?
Legacy British equipment, automotive restoration, and heritage plant.
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