Thread Types Explained
Understanding coarse vs fine threads, UNC, UNF, metric pitch, and how to identify the threads you need.
📖 10 min read • Includes thread identification charts • Printable reference tables
Thread Basics
Threads are the helical ridges wrapped around a fastener that allow it to be screwed into a mating part. Understanding thread terminology is essential for selecting the right fastener and ensuring proper fit.
Key Thread Terminology
| Term | Definition |
|---|---|
| Major Diameter | The largest diameter of the thread (outer edge of external threads, inner edge of internal threads) |
| Minor Diameter | The smallest diameter of the thread (root of external threads, crest of internal threads) |
| Pitch Diameter | The diameter at the midpoint of the thread where thread width equals the space between threads |
| Pitch | The distance from one thread crest to the next, measured parallel to the axis |
| TPI (Threads Per Inch) | The number of thread crests in one inch of length (inch fasteners) |
| Lead | The distance a fastener advances in one complete revolution (equals pitch for single-start threads) |
| Thread Angle | The angle between thread flanks (60° for unified and metric threads) |
| Crest | The peak or top of a thread |
| Root | The valley or bottom of a thread |
| Flank | The angled surface connecting crest to root |
How to Read Thread Callouts
Inch Thread Example: 1/4-20 UNC
- 1/4 = Major diameter (1/4 inch = 0.250")
- 20 = Threads per inch (TPI)
- UNC = Thread series (Unified National Coarse)
Metric Thread Example: M8-1.25
- M8 = Major diameter (8mm) — the "M" indicates metric
- 1.25 = Thread pitch (1.25mm between threads)
Need Thread Pitch Gauges?
Essential tools for identifying unknown threads
Coarse Thread vs Fine Thread
The most fundamental thread choice is between coarse and fine pitch. Each has distinct advantages that make it better suited for specific applications.
✓ Coarse Thread (UNC)
- Faster assembly and disassembly
- More tolerant of damaged threads
- Better for soft materials (less stripping)
- Less likely to cross-thread
- Better in dirty/gritty environments
- More resistant to galling
- Easier to tap by hand
- Lower cost to manufacture
- The default choice for most applications
✓ Fine Thread (UNF)
- Higher tensile strength (larger stress area)
- Better vibration resistance
- Finer adjustment capability
- Better for thin-wall applications
- Higher stripping resistance in hard materials
- Better thread engagement in short lengths
- Smaller tap drill (more material around hole)
- Tighter manufacturing tolerances
- Choose when strength or precision is critical
When to Use Each
| Application | Recommended | Why |
|---|---|---|
| General construction | Coarse | Faster installation, forgiving in field conditions |
| Automotive chassis | Fine | Vibration resistance, high strength requirements |
| Aluminum or plastic | Coarse | Resists stripping in soft materials |
| Hardened steel | Fine | Better engagement, less likely to strip |
| Outdoor/dirty environments | Coarse | Tolerates dirt and corrosion better |
| Precision adjustment | Fine | More turns per inch = finer control |
| Thin-wall tubing | Fine | More threads engage in short depth |
| Repeated assembly/disassembly | Coarse | Better wear resistance over time |
| Hydraulic fittings | Fine | Better seal, vibration resistance |
| Wood or sheet metal | Coarse | Better grip, faster driving |
Tensile Strength Comparison
Fine threads have approximately 10% greater tensile strength than coarse threads of the same diameter due to their larger stress area (more material at the root). Here's a comparison:
| Bolt Size | Coarse (UNC) Stress Area |
Fine (UNF) Stress Area |
Fine Thread Advantage |
|---|---|---|---|
| 1/4" | 0.0318 sq in | 0.0364 sq in | +14% |
| 3/8" | 0.0775 sq in | 0.0878 sq in | +13% |
| 1/2" | 0.1419 sq in | 0.1599 sq in | +13% |
| 3/4" | 0.3340 sq in | 0.3730 sq in | +12% |
| 1" | 0.6057 sq in | 0.6630 sq in | +9% |
Inch Thread Series (Unified National)
The Unified Thread Standard (UTS) is the dominant inch-based thread system used in the United States, Canada, and the United Kingdom. It was established in 1949 to standardize threads across North America and replace the older American National thread system.
Thread Series Designations
| Series | Full Name | Description | Common Uses |
|---|---|---|---|
| UNC | Unified National Coarse | Standard coarse pitch; most common thread series | General purpose, construction, machinery |
| UNF | Unified National Fine | Standard fine pitch; more threads per inch | Automotive, aerospace, precision equipment |
| UNEF | Unified National Extra Fine | Very fine pitch for special applications | Thin-wall tubing, instruments, aerospace |
| UNS | Unified National Special | Non-standard pitch for specific applications | Custom applications, legacy equipment |
| UN | Unified National (Constant Pitch) | Same pitch across all diameters (4, 6, 8, 12, 16, 20, 28, 32 TPI) | Large bolts, high-pressure applications |
Standard UNC and UNF Threads
These are the standard threads per inch (TPI) for common fastener sizes:
| Size | Major Diameter | UNC (Coarse) TPI |
UNF (Fine) TPI |
UNEF (Extra Fine) TPI |
|---|---|---|---|---|
| #0 | 0.060" | — | 80 | — |
| #1 | 0.073" | 64 | 72 | — |
| #2 | 0.086" | 56 | 64 | — |
| #3 | 0.099" | 48 | 56 | — |
| #4 | 0.112" | 40 | 48 | — |
| #5 | 0.125" | 40 | 44 | — |
| #6 | 0.138" | 32 | 40 | — |
| #8 | 0.164" | 32 | 36 | — |
| #10 | 0.190" | 24 | 32 | — |
| #12 | 0.216" | 24 | 28 | 32 |
| 1/4" | 0.250" | 20 | 28 | 32 |
| 5/16" | 0.3125" | 18 | 24 | 32 |
| 3/8" | 0.375" | 16 | 24 | 32 |
| 7/16" | 0.4375" | 14 | 20 | 28 |
| 1/2" | 0.500" | 13 | 20 | 28 |
| 9/16" | 0.5625" | 12 | 18 | 24 |
| 5/8" | 0.625" | 11 | 18 | 24 |
| 3/4" | 0.750" | 10 | 16 | 20 |
| 7/8" | 0.875" | 9 | 14 | 20 |
| 1" | 1.000" | 8 | 12 | 20 |
| 1-1/8" | 1.125" | 7 | 12 | 18 |
| 1-1/4" | 1.250" | 7 | 12 | 18 |
| 1-1/2" | 1.500" | 6 | 12 | 18 |
Metric Threads (ISO)
Metric threads follow the ISO metric screw thread standard, used worldwide and increasingly common in North America. The thread angle is 60° (same as unified threads), but sizes and pitch are expressed in millimeters.
How Metric Thread Callouts Work
- M10 = Major diameter is 10mm (the "M" prefix indicates metric)
- 1.5 = Thread pitch is 1.5mm (distance between threads)
Note: If only "M10" is shown without pitch, it implies the coarse pitch (1.5mm in this case).
Standard Metric Thread Sizes
| Size | Major Diameter | Coarse Pitch | Fine Pitch | Approx. Inch Equivalent |
|---|---|---|---|---|
| M3 | 3.0mm | 0.5mm | 0.35mm | ≈ #4 |
| M4 | 4.0mm | 0.7mm | 0.5mm | ≈ #8 |
| M5 | 5.0mm | 0.8mm | 0.5mm | ≈ #10 |
| M6 | 6.0mm | 1.0mm | 0.75mm | ≈ 1/4" |
| M8 | 8.0mm | 1.25mm | 1.0mm | ≈ 5/16" |
| M10 | 10.0mm | 1.5mm | 1.25mm, 1.0mm | ≈ 3/8" |
| M12 | 12.0mm | 1.75mm | 1.5mm, 1.25mm | ≈ 1/2" |
| M14 | 14.0mm | 2.0mm | 1.5mm | ≈ 9/16" |
| M16 | 16.0mm | 2.0mm | 1.5mm | ≈ 5/8" |
| M18 | 18.0mm | 2.5mm | 2.0mm, 1.5mm | ≈ 3/4" |
| M20 | 20.0mm | 2.5mm | 2.0mm, 1.5mm | ≈ 3/4" |
| M22 | 22.0mm | 2.5mm | 2.0mm, 1.5mm | ≈ 7/8" |
| M24 | 24.0mm | 3.0mm | 2.0mm | ≈ 1" |
| M30 | 30.0mm | 3.5mm | 2.0mm | ≈ 1-1/4" |
| M36 | 36.0mm | 4.0mm | 3.0mm | ≈ 1-1/2" |
Metric vs Inch: Quick Comparison
| Feature | Inch (Unified) | Metric (ISO) |
|---|---|---|
| Diameter unit | Inches (fractions or decimals) | Millimeters |
| Pitch expression | Threads per inch (TPI) | Millimeters between threads |
| Thread angle | 60° | 60° |
| Example coarse | 1/2-13 (13 TPI) | M12-1.75 (1.75mm pitch) |
| Example fine | 1/2-20 (20 TPI) | M12-1.25 (1.25mm pitch) |
| Primary use | USA, Canada, some UK | Worldwide (including USA imports) |
📥 Download Thread Charts PDF
Printable reference for UNC, UNF, and metric threads
How to Identify Thread Type
Identifying an unknown thread requires determining three things: diameter, pitch, and thread system (inch or metric). Here's how to figure it out:
Step 1: Measure the Major Diameter
Use calipers to measure across the outside of the threads (the major diameter). This tells you the nominal size:
- If the measurement is a clean metric number (6mm, 8mm, 10mm), it's likely metric
- If it matches an inch fraction (0.250" = 1/4", 0.375" = 3/8"), it's likely inch
- Note: Actual measurement may be slightly smaller than nominal due to thread depth
Step 2: Determine the Pitch
Using a Thread Pitch Gauge
The most accurate method. Thread pitch gauges have multiple blades with different pitches—find the one that fits perfectly into the threads. Gauges come in inch (TPI) and metric (mm pitch) versions.
Using a Known Nut
Try threading a known nut onto the bolt. If it goes on smoothly with no binding or wobbling, you've found a match. Keep a set of nuts in common sizes for this purpose.
Counting Threads
Place a ruler along the threads and count how many thread peaks fit in one inch. This gives you the TPI for inch threads. For metric, measure the distance between thread peaks in millimeters.
Using Calipers
Measure across 10 threads and divide by 10 to get the pitch in mm. Or measure 1" of threads and count the peaks for TPI. This averaging improves accuracy.
Step 3: Confirm the Thread System
Once you have diameter and pitch, check if it matches a standard thread:
Example: You measure 0.375" diameter and count 16 TPI
→ Check the UNC chart: 3/8" UNC = 16 TPI ✓ Match!
Example: You measure 8mm diameter and 1.25mm pitch
→ Check the metric chart: M8 coarse = 1.25mm ✓ Match!
Thread Classes (Fit & Tolerance)
Thread class defines the fit between mating threads—how tight or loose they are. It's determined by the manufacturing tolerances applied to the pitch diameter.
Unified Thread Classes (Inch)
Classes are designated by a number (1, 2, or 3) followed by A (external/bolt) or B (internal/nut):
| Class | Fit Type | Tolerance | Common Uses |
|---|---|---|---|
| 1A / 1B | Loose fit | Widest tolerance | Quick assembly, dirty conditions, hot-dip galvanized |
| 2A / 2B | Free fit | Standard tolerance | Most commercial fasteners — the default |
| 3A / 3B | Close fit | Tightest tolerance | Precision equipment, aerospace, no allowance for play |
Metric Thread Classes (ISO)
Metric threads use a tolerance class system with a number and letter. The number indicates tolerance grade (lower = tighter), and the letter indicates position (deviation from basic):
| Type | Tolerance Class | Description |
|---|---|---|
| External (bolt) | 6g | Standard, equivalent to Class 2A (most common) |
| Internal (nut) | 6H | Standard, equivalent to Class 2B (most common) |
| External (bolt) | 4g | Close fit, equivalent to Class 3A |
| Internal (nut) | 4H | Close fit, equivalent to Class 3B |
A full metric callout might look like: M8 × 1.25-6g (8mm diameter, 1.25mm pitch, standard external thread tolerance).
Special Thread Types
Beyond standard fastener threads, there are several specialized thread forms designed for specific applications:
NPT — National Pipe Thread (Tapered)
NPT threads are tapered at 1°47' (3/4" per foot) to create a seal as the threads tighten. Used for plumbing, pneumatics, and hydraulics where leak-tight connections are needed.
- Thread angle: 60°
- Taper: 3/4" per foot (1:16 ratio)
- Sealing: Thread sealant (Teflon tape or pipe dope) required
- Sizes: Nominal pipe size (NPS), not actual diameter
- Related: NPTF (dryseal, no sealant needed), NPS (straight/parallel pipe)
BSPT / BSP — British Standard Pipe
The British equivalent to NPT, but not interchangeable. Uses a 55° thread angle instead of 60°.
- BSPT: British Standard Pipe Tapered (sealing)
- BSPP: British Standard Pipe Parallel (requires washer/O-ring seal)
- Thread angle: 55° (Whitworth form)
- Common in: UK, Europe, Asia, Australia
ACME Threads
A trapezoidal thread form designed for power transmission rather than fastening. Used in lead screws, vises, and linear motion systems.
- Thread angle: 29° (between flanks)
- Shape: Trapezoidal (flat crests and roots)
- Advantages: Stronger than V-threads, efficient power transmission, wear-resistant
- Uses: Lead screws, CNC machines, clamps, jacks
Buttress Threads
Asymmetrical threads designed for high axial load in one direction. One flank is nearly perpendicular, the other is angled.
- Thread angles: 7° load flank, 45° trailing flank
- Uses: Vise screws, artillery breech blocks, hydraulic press screws, bottle caps
Left-Hand Threads
Standard threads are right-hand (clockwise to tighten). Left-hand threads tighten counterclockwise and are marked "LH" in callouts.
- Uses: Turnbuckles (one end), bicycle left pedals, some gas fittings, rotating shafts (prevents loosening)
- Identification: Threads slope up to the left; often marked with groove on hex flats
- Callout example: 1/2-13 UNC-2A LH
Thread Comparison Summary
| Thread Type | Thread Angle | Profile | Primary Use |
|---|---|---|---|
| UN/UNC/UNF | 60° | V-thread (sharp) | Fastening |
| ISO Metric | 60° | V-thread (sharp) | Fastening |
| NPT | 60° | V-thread (tapered) | Pipe sealing (US) |
| BSP | 55° | V-thread (Whitworth) | Pipe fittings (UK/Europe) |
| ACME | 29° | Trapezoidal | Power screws, linear motion |
| Buttress | 7°/45° | Asymmetric | High unidirectional load |
Shop Fasteners by Thread Type
UNC, UNF, metric, and specialty threads in stock
Quick Reference: Tap Drill Sizes
When threading a hole, you need to drill it first with a tap drill—slightly smaller than the major diameter to leave material for the threads. Here are the most common sizes:
Inch (UNC) Tap Drill Chart
| Thread Size | TPI | Tap Drill (75% thread) | Decimal |
|---|---|---|---|
| #4-40 | 40 | #43 | 0.089" |
| #6-32 | 32 | #36 | 0.106" |
| #8-32 | 32 | #29 | 0.136" |
| #10-24 | 24 | #25 | 0.149" |
| #10-32 | 32 | #21 | 0.159" |
| 1/4-20 | 20 | #7 | 0.201" |
| 1/4-28 | 28 | #3 | 0.213" |
| 5/16-18 | 18 | F | 0.257" |
| 5/16-24 | 24 | I | 0.272" |
| 3/8-16 | 16 | 5/16" | 0.312" |
| 3/8-24 | 24 | Q | 0.332" |
| 7/16-14 | 14 | U | 0.368" |
| 1/2-13 | 13 | 27/64" | 0.422" |
| 1/2-20 | 20 | 29/64" | 0.453" |
| 5/8-11 | 11 | 17/32" | 0.531" |
| 3/4-10 | 10 | 21/32" | 0.656" |
Metric (Coarse) Tap Drill Chart
| Thread Size | Pitch | Tap Drill (mm) | Tap Drill (inch approx) |
|---|---|---|---|
| M3 | 0.5mm | 2.5mm | #39 (0.099") |
| M4 | 0.7mm | 3.3mm | #30 (0.128") |
| M5 | 0.8mm | 4.2mm | #19 (0.166") |
| M6 | 1.0mm | 5.0mm | #8 (0.199") |
| M8 | 1.25mm | 6.8mm | H (0.266") |
| M10 | 1.5mm | 8.5mm | R (0.339") |
| M12 | 1.75mm | 10.2mm | Y (0.404") |
| M14 | 2.0mm | 12.0mm | 15/32" (0.469") |
| M16 | 2.0mm | 14.0mm | 35/64" (0.547") |
| M18 | 2.5mm | 15.5mm | 39/64" (0.609") |
| M20 | 2.5mm | 17.5mm | 11/16" (0.688") |
Key Takeaways
- Coarse thread (UNC) is the default — use it unless you have a specific reason for fine
- Fine thread (UNF) for vibration resistance, high strength, or thin materials
- Metric and inch threads are NOT interchangeable — verify before forcing
- Use a thread pitch gauge to identify unknown threads accurately
- Class 2A/2B (6g/6H) is standard tolerance for commercial fasteners
- Pipe threads (NPT) are tapered and seal on the threads — don't confuse with fastener threads
- 75% thread engagement is standard and provides full strength
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