Are you confused by common terms such as “thread,” “screw thread,” “connection,” or “fitting,” as well as the wide range of specifications available?
SJ Gauge offers four primary thread connection types: NPT, PT, G, and M. Are you wondering which option best suits your specific application requirements? In the following article, we will provide a detailed introduction to these commonly used thread types. If you are curious about the differences between them, please continue reading. Alternatively, feel free to contact our professional sales team—we will be happy to recommend the most suitable customized measurement solution for your application.
1. What Is a Thread? What Is Its Function?
A thread, also commonly referred to as a connection or fitting, is the interface used to connect a pressure gauge to a pressure source. It consists of a male thread (external thread) and a female thread (internal thread), which are tightened together to form a secure connection.
The primary function of a thread is to ensure that pressure or media can be transmitted from the pressure source to the gauge under a completely sealed condition. In some cases, additional sealing accessories such as PTFE tape, O-rings, or gaskets are used to enhance sealing performance.
Therefore, when selecting a thread size, it is essential to ensure compatibility with the corresponding instrument connection. When purchasing instruments such as pressure gauges or thermometers, it is also recommended to provide the required thread specifications to the supplier in advance.
2. What Problems Can Occur If the Wrong Thread Type Is Used?
Before confirming the correct thread connection type, do not attempt to connect the thread to your instrument. Doing so may result in instrument damage, production downtime, and even reduced operational efficiency or financial loss.
In addition, visual inspection alone is not sufficient to accurately determine whether a thread connection type is correct.
Why Is It Important to Identify the Correct Thread Type?
When attempting to connect two different thread types, differences in pitch or diameter may prevent proper engagement, making it impossible to fully tighten the connection. If forced installation is attempted, it may cause deformation of the thread angle, jamming, or complete seizure. Even if the connection is later removed from the pipeline, a proper seal may no longer be achievable.
These issues can lead to media leakage, contamination, or equipment failure. Fortunately, by correctly identifying the appropriate thread type, these risks can be significantly reduced.
In the next section, SJ Gauge will guide you through different thread types and how to measure them.
3. Two Essential Measurement Tools You Must Know Before Selecting Threads
a. Vernier Caliper
A vernier caliper, also known as a “vernier scale” or “slide caliper,” is a commonly used industrial measuring tool for determining length, inner diameter, outer diameter, and depth. It consists of a main scale and a sliding vernier scale that moves along the main body.
The main scale is typically marked in millimeters. Depending on the scale division, vernier calipers are classified into 10-division, 20-division, or 50-division types, commonly referred to as 0.1 mm, 0.05 mm, and 0.02 mm resolution calipers, respectively.
Today, vernier calipers are available in various forms. In addition to the traditional mechanical type shown above, there are also dial calipers (with a pointer display) and digital calipers (with LCD readouts).
b. Thread Pitch Gauge
A thread pitch gauge, also known as a “screw pitch gauge” or “thread gauge,” is used to assist in measuring thread specifications. By matching the gauge teeth against the thread profile and finding the perfect fit, users can identify the correct thread pitch.
The most commonly used thread standards on the market include American (Imperial), British, and Metric systems, each with its corresponding thread gauge specifications.
4. Identifying Threads in Four Simple Steps
a. Determine Whether the Thread Is Parallel or Tapered
Threads can be classified as either parallel (straight) or tapered. If the thread diameter gradually increases or decreases along its length, it is a tapered thread. If the diameter remains constant, it is a parallel thread.
If it is difficult to distinguish visually, a vernier caliper can be used for assistance. Simply measure both ends of the thread and observe the gap or alignment. If a triangular gap appears (indicating non-parallel sides), it is a tapered thread. If both sides remain parallel with no visible gap, it is a parallel thread.
b. Determine the Type of Thread and Pitch
Thread specifications are typically measured using common standards such as American (NPT/UN), British (BSP), and Metric (M) systems. The correct thread type is identified by matching the thread gauge that provides the best fit.
There are two primary ways to express thread pitch:
- Metric system: Measures the distance between two adjacent thread peaks
(e.g., the distance between peaks is 1.5 mm) - Imperial / British system: Measures the number of threads per inch (TPI)
(e.g., 14 threads per inch)
c. Determine the Size of the Connector
Use a vernier caliper to measure the outer diameter of the thread by clamping both ends of the thread. This will provide an approximate value of the fitting size. Then, compare the measured value with the reference table below to identify the closest corresponding size.
Common industry terms for thread sizes are also listed for reference:
| Nominal Size | Thread Size | Outer Diameter (mm) |
|---|---|---|
| 1/8″ (One-Eighth) | 1/8 | 9.7 – 10.3 |
| 1/4″ (Quarter) | 1/4 | 13.1 – 13.6 |
| 3/8″ (Three-Eighths) | 3/8 | 16.6 – 17.1 |
| 1/2″ (Half) | 1/2 | 20.9 – 21.3 |
| 3/4″ (Three-Quarter) | 3/4 | 26.4 – 26.6 |
d. Compare with Thread Standards (Type / Pitch / Size)
You can refer to the following compiled information and tables to identify the most common thread types, including American, British, and Metric standards, by comparing thread form (parallel/tapered), pitch, and size.
d-1. American Standard Straight Threads (SAE=UN=UNF)
The most common straight thread form used in the United States is the UN/UNF thread (Unified National Fine), also referred to as SAE threads (Society of Automotive Engineers). The table below lists several commonly used SAE straight thread sizes.
The standard designation format for American straight threads is:
Size – Threads per Inch – Series
For example: 3/4–16 UN/UNF
| American Standard Straight Threads SAE=UN=UNF | |||||
| Thread Size | Threads per inch (TPI) | Thread diameter (mm) | Thread angle | ||
| Major diameter | Simple pitch diameter | Minor diameter | |||
| 5/16 | 24 | 7.938 | 7.249 | 6.792 | 60° |
| 7/16 | 20 | 11.113 | 10.287 | 9.738 | |
| 1/2 | 20 | 12.7 | 11.875 | 11.325 | |
| 9/16 | 18 | 14.288 | 13.325 | 12.559 | |
| 3/4 | 16 | 19.05 | 18.019 | 17.331 | |
| 1-1/16 | 12 | 26.988 | 25.613 | 24.695 | |
| 1-5/16 | 12 | 33.338 | 31.963 | 31.046 | |
| Size-TPI, Type e.g. 3/4-16, UN | |||||
d-2. American Tapered Threads (NPT / NPTF)
NPT (National Pipe Taper) threads, formally known as American National Standard Taper Pipe Threads, are one of the most commonly used tapered thread standards in the United States.
NPTF (Dryseal) threads, also known as American Standard Taper Pipe Threads (Dryseal), are designed to provide a tighter seal without the need for additional sealing materials. Although not officially recommended by the National Fluid Power Association (NFPA), they are still widely used in various industries.
The main difference between NPT and NPTF threads lies in the thread crest and root design. NPT threads do not form a perfect metal-to-metal seal at the crest, which may result in minor leakage after assembly. Therefore, sealing materials such as PTFE tape or sealant are typically required. In contrast, NPTF threads are designed to achieve a “dry seal” through controlled interference.
Designation Format:
Size – Threads per Inch – Type
e.g., 1/4–18 NPT
| American Standard Tapered Threads NPT / NPTF | |||||
| Thread Size | Threads per inch (TPI) | Thread diameter (mm) | Thread angle | ||
| Major diameter | Simple pitch diameter | Minor diameter | |||
| 1/16 | 27 | 7.894 | 7.142 | 6.398 | 60° |
| 1/8 | 27 | 10.242 | 9.489 | 8.737 | |
| 1/4 | 18 | 13.616 | 12.487 | 11.358 | |
| 3/8 | 18 | 17.055 | 15.926 | 14.797 | |
| 1/2 | 14 | 21.224 | 19.772 | 18.321 | |
| 3/4 | 14 | 26.569 | 25.117 | 23.666 | |
| 1 | 11.5 | 33.228 | 31.461 | 29.694 | |
| 1-1/4 | 11.5 | 41.985 | 40.218 | 38.454 | |
| 1-1/2 | 11 | 48.054 | 46.287 | 44.52 | |
| 2 | 11 | 60.092 | 58.325 | 56.558 | |
| Size-TPI, Type e.g. 1/4-18, NPT | |||||
d-3. British Straight Threads (BSPP = BSP = PF = G)
British thread standards are established by the British Standards Institution (BSI). The most common names for British straight threads include G thread, BSPP (British Standard Pipe Parallel), BSPM, and BSP.
The term BSPF (British Standard Pipe Fitting) is an older designation for BSPP threads. It was introduced to reduce confusion between BSPP (parallel threads) and BSPT (tapered threads) in earlier usage.
Designation Format:
G Size – Threads per Inch (TPI)
e.g., G 1/8–28
| British Standard Parallel Threads BSPP=BSP=PF=G | |||||
| Thread Size | Threads per inch (TPI) | Thread diameter (mm) | Thread angle | ||
| Major diameter | Simple pitch diameter | Minor diameter | |||
| 1/16 | 28 | 7.732 | 7.142 | 6.561 | 55° |
| 1/8 | 28 | 9.782 | 9.147 | 8.566 | |
| 1/4 | 19 | 13.157 | 12.301 | 11.455 | |
| 3/8 | 19 | 16.662 | 15.806 | 14.95 | |
| 1/2 | 14 | 20.955 | 19.793 | 18.631 | |
| 5/8 | 14 | 22.911 | 21.749 | 20.587 | |
| 3/4 | 14 | 26.441 | 25.279 | 24.117 | |
| 7/8 | 14 | 30.201 | 29.039 | 27.877 | |
| 1 | 11 | 33.249 | 31.77 | 30.291 | |
| 1-1/8 | 11 | 37.897 | 36.418 | 34.939 | |
| 1-1/4 | 11 | 41.91 | 40.431 | 38.952 | |
| 1-1/2 | 11 | 47.803 | 46.324 | 44.845 | |
| 1-3/4 | 11 | 53.746 | 52.267 | 50.788 | |
| 2 | 11 | 59.614 | 58.135 | 56.656 | |
| G Size-TPI e.g. G 1/8-28 | |||||
d-4. British Tapered Threads (BSPT = PT = R = RC = ZG)
British tapered threads are commonly referred to as BSPT (British Standard Pipe Taper). In modern standards, the designation “R” thread is increasingly used, although BSPT remains a widely recognized and commonly used term in industry.
Designation Format:
R Size – Threads per Inch (TPI)
e.g., R 1/2–14
| British Standard Tapered Threads BSPT=PT=R=RC=ZG | |||||
| Thread Size | Threads per inch (TPI) | Thread diameter (mm) | Thread angle | ||
| Major diameter | Simple pitch diameter | Minor diameter | |||
| 1/16 | 28 | 7.723 | 7.142 | 6.561 | 55° |
| 1/8 | 28 | 9.728 | 9.147 | 8.566 | |
| 1/4 | 19 | 13.157 | 12.3 | 11.445 | |
| 3/8 | 19 | 16.662 | 15.81 | 14.95 | |
| 1/2 | 14 | 20.995 | 19.79 | 18.631 | |
| 3/4 | 14 | 26.441 | 25.28 | 24.117 | |
| 1 | 11 | 33.249 | 31.77 | 30.291 | |
| 1-1/4 | 11 | 41.91 | 40.43 | 38.952 | |
| 1-1/2 | 11 | 47.803 | 46.32 | 44.845 | |
| 2 | 11 | 59.641 | 58.14 | 56.656 | |
| R Size-TPI e.g. R 1/2-14 | |||||
d-5. ISO Metric Threads (M)
ISO metric threads are one of the most widely recognized and internationally standardized thread types. ISO 68-1 defines the fundamental design principles of metric threads. These threads feature a symmetrical V-shaped profile with a 60° thread angle, and both internal and external threads are parallel.
Metric threads are designated by the letter “M”. There are two main pitch types: coarse pitch and fine pitch. Coarse threads are the most commonly used and follow standardized pitch values, while fine threads have smaller pitch sizes.
The thread designation clearly indicates the diameter and pitch. For example:
M4 × 0.5 means the thread has a nominal diameter of 4 mm and a pitch of 0.5 mm.
If the pitch is not specified, it is assumed to be the standard coarse pitch defined in ISO 261 / ISO 262. For example, M8 implies a coarse thread. Similarly, if only M20 is provided, it is understood as a coarse pitch thread.
Designation Format:
M Size × Pitch
e.g., M14 × 1.5
| ISO Metric Threads M | |||||
| Thread Size | Pitch | Thread diameter (mm) | Thread angle | ||
| Major diameter | Simple pitch diameter | Minor diameter | |||
| M6 | 1 | 6 | 5.35 | 4.917 | 60° |
| M8 | 8 | 7.35 | 6.917 | ||
| M10 | 10 | 9.35 | 8.917 | ||
| M12 | 1.5 | 12 | 11.03 | 10.376 | |
| M14 | 14 | 13.03 | 12.376 | ||
| M16 | 16 | 15.03 | 14.376 | ||
| M18 | 18 | 17.03 | 16.376 | ||
| M20 | 20 | 19.03 | 18.376 | ||
| M22 | 22 | 21.03 | 20.376 | ||
| M24 | 24 | 23.03 | 22.376 | ||
| M27 | 2 | 27 | 25.7 | 24.835 | |
| M30 | 30 | 28.7 | 27.835 | ||
| M33 | 33 | 31.7 | 30.835 | ||
| M36 | 36 | 34.7 | 33.835 | ||
| M Size x Pitch e.g. M 14 x 1.5 | |||||
5. Four Steps to Identify Thread Specifications
| Thread Standard | Step 1 | Step 2 | Step 3 | Step 4 |
| Determine the Type of Thread | Determine the Pitch of Thread | Determine the Size of Thread | Determine the Thread Standard | |
| American Standard Straight Threads (SAE=UN=UNF) | Straight | 12、14、16 18、20、24 | Measuring using: Vernier caliper Dial caliper Digital caliper | Size-TPI, Type e.g. 3/4-16, UN |
| American Standard Tapered Threads (NPT / NPTF) | Tapered | 11-1/2、14 18、27 | Size-TPI, Type e.g. 1/4-18, NPT | |
| British Standard Parallel Threads (BSPP=BSP=PF=G) | Straight | 11、14 19、28 | G Size-TPI e.g. G 1/8-28 | |
| British Standard Tapered Threads (BSPT=PT=R=RC=ZG) | Tapered | 11、14 19、28 | R Size-TPI e.g. R 1/2-14 | |
| ISO Metric Threads M | Straight | 1.0、1.5、2.0 | M Size x Pitch e.g. M 14 x 1.5 |
Now that you’ve learned how to select the thread type and size, are you wondering how to install the instrument? Click the link below to learn about the installation process and related precautions!
- Top Six Common Reasons of Pressure Equipment Failure
- Key Safety Points for Installing Pressure Gauges
Credit and Reference:
- ASME: Pipe Threads, General Purpose, Inch
- ISO 68-1:1998(en) ISO general purpose screw threads — Basic profile — Part 1: Metric screw threads
- ISO/DIS 6149-1(en) Connections for hydraulic fluid power and general use — Ports and stud ends with ISO 261 metric threads and O-ring sealing — Part 1: Ports with truncated housing for O-ring seal
- ISO 262:1998 ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts
