Fastener Thread Types Guide
Overview
Understanding thread types is fundamental to proper fastener selection. The wrong thread won't engage, won't hold, or may strip prematurely. This guide covers the major thread systems, their characteristics, and how to identify and select the right thread for your application.
Major Thread Systems
1. Unified Thread Standard (UTS) — US/Inch
| Thread Series | Abbreviation | Description |
|---|---|---|
| Coarse | UNC | Standard, most common in US |
| Fine | UNF | Higher strength, adjustment |
| Extra Fine | UNEF | Thin walls, aerospace |
| Constant Pitch | UN | Various pitches (e.g., 8-UN) |
Thread Angle: 60°
Designation: Diameter-TPI (e.g., 1/4-20 UNC)
2. ISO Metric Thread — International
| Thread Series | Abbreviation | Description |
|---|---|---|
| Coarse | M | Standard, most common globally |
| Fine | M×P | Tighter pitch (e.g., M8×1.0) |
Thread Angle: 60°
Designation: M + Diameter × Pitch (e.g., M8×1.25)
3. British Standard Threads
| Thread Type | Abbreviation | Origin/Use |
|---|---|---|
| BSW | British Standard Whitworth | Original standard, 55° angle |
| BSF | British Standard Fine | Fine pitch version of BSW |
| BSP | British Standard Pipe | Pipe threads (parallel and tapered) |
| BA | British Association | Small precision, instruments |
Thread Angle: 55° (Whitworth and BSP)
UNC vs UNF — Coarse vs Fine
Unified Coarse (UNC)
| Size | Threads Per Inch |
|---|---|
| #6 | 32 |
| #8 | 32 |
| #10 | 24 |
| 1/4" | 20 |
| 5/16" | 18 |
| 3/8" | 16 |
| 1/2" | 13 |
| 5/8" | 11 |
| 3/4" | 10 |
| 1" | 8 |
Advantages:
- Faster assembly (fewer turns)
- More forgiving (less sensitive to damage)
- Better for softer materials (less stripping)
- Easier to tap
Unified Fine (UNF)
| Size | Threads Per Inch |
|---|---|
| #6 | 40 |
| #8 | 36 |
| #10 | 32 |
| 1/4" | 28 |
| 5/16" | 24 |
| 3/8" | 24 |
| 1/2" | 20 |
| 5/8" | 18 |
| 3/4" | 16 |
| 1" | 12 |
Advantages:
- Higher tensile strength (larger minor diameter)
- Finer adjustment capability
- Better for thin materials
- Higher shear strength
When to Use Each
| Application | Use UNC | Use UNF |
|---|---|---|
| General assembly | ✓ | |
| Cast iron/aluminum | ✓ | |
| Production (speed) | ✓ | |
| Thin walls | ✓ | |
| Fine adjustment | ✓ | |
| Vibration-prone | ✓ | |
| Maximum strength | ✓ |
Metric Coarse vs Fine
Metric Coarse (Standard)
| Size | Pitch (mm) |
|---|---|
| M3 | 0.5 |
| M4 | 0.7 |
| M5 | 0.8 |
| M6 | 1.0 |
| M8 | 1.25 |
| M10 | 1.5 |
| M12 | 1.75 |
| M16 | 2.0 |
| M20 | 2.5 |
| M24 | 3.0 |
Metric Fine (Examples)
| Size | Standard Pitch | Fine Pitch |
|---|---|---|
| M8 | 1.25mm | 1.0mm |
| M10 | 1.5mm | 1.25mm or 1.0mm |
| M12 | 1.75mm | 1.5mm or 1.25mm |
| M16 | 2.0mm | 1.5mm |
| M20 | 2.5mm | 2.0mm or 1.5mm |
Metric Fine Designation: Always includes pitch (e.g., M10×1.25)
Metric Coarse: Pitch often omitted (e.g., M10 = M10×1.5)
Thread Form Comparison
| Feature | Unified (US) | Metric (ISO) | Whitworth (BS) |
|---|---|---|---|
| Thread Angle | 60° | 60° | 55° |
| Root | Flat or rounded | Rounded preferred | Rounded |
| Crest | Flat | Flat | Rounded |
| Units | TPI (per inch) | mm pitch | TPI |
| Origin | US/Canada | International (ISO) | British |
Specialty Thread Types
Acme Threads
- Angle: 29° included
- Use: Power transmission, lead screws
- Characteristic: Trapezoidal profile, high load capacity
Buttress Threads
- Angle: 7° (load side), 45° (clearance side)
- Use: High axial load in one direction
- Characteristic: Asymmetric profile
Square Threads
- Angle: 0° (parallel sides)
- Use: Maximum efficiency power screws
- Characteristic: Difficult to manufacture
Pipe Threads
| Type | Code | Use |
|---|---|---|
| NPT | National Pipe Tapered | Sealing, plumbing |
| NPS | National Pipe Straight | Mechanical joints |
| BSPT | British Standard Pipe Tapered | UK/Commonwealth sealing |
| BSPP | British Standard Pipe Parallel | UK/Commonwealth mechanical |
NPT vs NPS: NPT has 1°47' taper and creates seal. NPS is straight (requires sealant).
Thread Identification
Visual Identification Steps
1. Measure outside diameter — Major diameter
2. Count threads per inch (inch) or measure pitch (metric)
3. Check thread angle — 60° (US/Metric) or 55° (Whitworth)
4. Try thread gauge — Confirms type
Thread Gauge Types
| Gauge Type | Measures |
|---|---|
| Thread pitch gauge | TPI or mm pitch |
| Go/No-Go plug gauge | Internal threads |
| Go/No-Go ring gauge | External threads |
| Thread micrometers | Pitch diameter |
Quick Identification Tips
- Even TPI (20, 24, 32) → Likely UNC/UNF
- Odd-looking pitch → Check if metric
- 55° angle → Likely Whitworth
- "M" marking → Metric
Thread Classes and Fits
Unified (US) Thread Classes
| Class | Fit | Use |
|---|---|---|
| 1A/1B | Loose | Dirty environments, quick assembly |
| 2A/2B | Standard | General purpose |
| 3A/3B | Tight | Precision, aerospace |
A = External (bolt), B = Internal (nut)
Metric Thread Classes
| Position | Fit |
|---|---|
| 6H/6g | Standard |
| 6G/6g | Close fit |
| 5H/4g | Transition (tighter) |
Lowercase = external, Uppercase = internal
Thread Pitch Measurement
Measuring TPI (Inch)
1. Place thread pitch gauge against threads
2. Match gauge teeth to thread peaks
3. Read TPI from gauge
Measuring Metric Pitch
1. Measure from peak to peak of adjacent threads
2. Use calipers or pitch gauge
3. Result in millimeters (e.g., 1.25mm)
Conversion
- Metric pitch = 25.4 ÷ TPI
- TPI = 25.4 ÷ Metric pitch
Example: 1/4-20 → 25.4 ÷ 20 = 1.27mm pitch (close to M6×1.0 but NOT interchangeable)
Common Thread Mistakes
| Mistake | Problem | Solution |
|---|---|---|
| Mixing metric/inch | Won't engage or strips | Verify thread type |
| UNC nut on UNF bolt | Threads don't match | Check TPI |
| BSP on NPT | Leaks (different angles) | Confirm standard |
| Wrong thread class | Too tight/loose | Specify class |
| Cross-threading | Strips threads | Start by hand |
Thread Compatibility Warning
DO NOT interchange:
- Metric and inch (even if similar diameter)
- UNC and UNF (same diameter, different pitch)
- NPT and BSP (different thread angle)
- Whitworth and Unified (55° vs 60°)
May seem to work but will fail:
- M8×1.25 with 5/16-18 (similar diameter, different pitch)
- M12×1.75 with 1/2-13 (similar but not interchangeable)
Thread Selection Guide
| Application | Recommended Thread |
|---|---|
| General US industrial | UNC |
| General international | Metric coarse |
| Fine adjustment | UNF or metric fine |
| Thin materials | UNF or metric fine |
| Soft materials | UNC or metric coarse |
| Pipe connections (seal) | NPT or BSPT |
| Power transmission | Acme |
| High axial load | Buttress |
FAQ
Q: How do I know if a bolt is metric or inch?
A: Measure the diameter and count TPI. Metric threads have pitch in mm and typically show "M" markings. When in doubt, use a thread gauge.
Q: Can I put a metric nut on an inch bolt if they're the same diameter?
A: No. Even if the diameter is close, the thread pitch and form will differ. They may seem to engage but will strip or fail.
Q: Why are there so many thread standards?
A: Different standards evolved in different countries before international standardization. Legacy equipment still uses old standards.
Q: What does "pitch" mean in metric vs inch?
A: Metric pitch = distance between threads (in mm). Inch threads use TPI (threads per inch) — it's the inverse measurement.
Q: Is coarse or fine thread stronger?
A: Fine threads have higher tensile strength (larger minor diameter) but coarse threads are more resistant to stripping in softer materials.
Need help identifying a thread? Contact our technical team or use our thread identification guide.