Pipe threads are a surprisingly complex topic — and an important one as well.
Worse still, choosing the wrong pipe thread type or size could cause irreparable damage to the components in use, costing additional time and money to rectify the situation.
This guide will look at the science behind pipe threads and the various standards and types of pipe thread common throughout the world.
We’ll also provide tips and tricks for determining your proper pipe thread type and best practices for safely using threaded pipe in your operations.
How Pipe Thread Works
Threads can take on various shapes and sizes, offering a tight seal for nearly any piped material, including air, gas, liquids, hydraulic fluids, slurries, and more.
Once reserved for brass and steel, you can find threaded pipe in a variety of materials today, including bronze, cast iron, nylon, PTFE, and PVC.
Depending on the thread type used, the connection may require additional treatment, such as tapes, coatings, or sealants, to lubricate or reinforce the seal.
Pipe Thread Standards
Pipe thread standards define thread pitch, thread angle, and thread dimensions, allowing for increased consistency across manufacturers and product lines.
Common standards include:
- National Pipe Thread or American National Standard Pipe Thread (NPT)
- British Standard Pipe (BSP)
- Japanese Industrial Standard (JIS) Tapered Pipe Thread (PT)
- SAE (SAE International) Straight Thread
- Metric Tapered/Parallel Thread (M)
As you can likely tell, your geographic location (or where the pipe or component in question was manufactured) will often impact the standard used.
To further complicate things, while various standards are often incompatible, each of these standards can also have different thread types that are not compatible even within the same thread standard.
While they might appear to thread together in some cases, in most, you won’t achieve a proper seal.
You might even irreparably damage the pipe, threads, or components joined. Precisely understanding what thread type and size you’re dealing with is essential.
We’ll be looking at the National Pipe Thread standard for this guide — the most widely used standard in the US and Canada in accordance to ASME B1.20.1.
Additional thread types within the NPT standard include:
- NPT: National Pipe Tapered
- NPS: National Pipe Straight
- NPSC: National Pipe Straight-Coupling
- NPSF: National Pipe Straight-Fuel
- NPSH: National Pipe Straight-Hose
- NPSI: National Pipe Straight-Intermediate
- NPSL: National Pipe Straight-Locknut
- NPSM: National Pipe Straight-Mechanical
- NPTF: National Pipe Taper-Fuel
- NPTR: National Pipe Taper-Railing
- PTF-SAE SHORT: Pipe taper, fuel, SAE, short
Determining Pipe Thread Type
If you want to ensure an effective fit and seal, it’s critical to know the exact type of threads with which you’re working.
In many cases, the fitting, pipe ends, ports, or other terminating locations will include a stamp that describes standards and measurements.
However, if you don’t see any information, you can use a few standard tools to gather additional details and limit thread options to a handful of choices.
- A calliper: Callipers are typically affordable and offer greater accuracy and ease of use. However, a steel straight rule can work as well.
- Pitch gauges: Laying these in the thread makes it easy to find a perfect measure of the threads-per-inch measurement of your pipe, fitting, or other components.
With these tools and some basic observations, you can gather information about the component in question and determine the thread type.
Things to consider include:
Is the thread male or female?
Male threads are on the outside of the piece, while female threads are on the inside.
The designation will have little influence on performance but is an essential part of ensuring that connections are possible and secure.
Do you have the right schedule of pipe?
Is the thread tapered or parallel?
Tapered threads subtly reduce in diameter as they extend from the end of the pipe or component.
The tapered shape allows the pipe to establish an effective seal using metal-to-metal wedging and compression.
NPT standards typically require a 3/4-inch decrease in diameter over 1 foot of distance.
If you cannot detect a taper through visual inspection, using a calliper to measure the first, middle, and final threads should make any taper apparent.
Parallel threads (also known as straight threads) are all equal and typically rely on an o-ring, gasket, washer, thread tape, or other treatment to establish an effective seal.
What is the thread’s pitch size?
The way you find this measurement varies slightly depending on the standard in use.
For most thread types, the pitch size is the number of threads in an inch of threaded pipe or component.
However, by metric standards, the pitch size describes the distance between threads.
Both details are easily found using pitch gauges or through visual observation and calliper measurements.
Finally, you’ll want to use the calliper to get the outside diameter on male threads and the inside diameter on female threads.
The pipe diameter gives you an excellent starting point for determining the nominal pipe size of your threading.
You might find that your measurements don’t line up precisely with standards with most nominal sizes.
These minor discrepancies are typical of nominal sizing.
However, in most cases, you’ll find an obvious answer that correlates with your findings.
Putting It All Together
With the information gathered, you can then compare your findings to the sizing charts for each standard.
In most cases, starting with the most common standard in the region in which the pipe or component is manufactured is a good starting point.
However, if you don’t find a match, check the standards expected in the regions the pipe or component was designed to be used in.
We’ve provided an example of a typical NPT basic dimensions chart below.
However, most standards offer tables that you can easily find with a quick search in your preferred search engine.
Pipe Thread Chart
ASME B1.20.1 (NPT/API)
BASIC DIMENSIONS OF AMERICAN NATIONAL STANDARD TAPER THREADS, NPT (ASME B1.20.1)
|NPS||Number of Threads Per Inch||Pitch of Thread||Depth of Thread||Truncation, max||Pitch diameter at plane of hand-tight engagement||Length from end of pipe to plane of hand- tight engagement||Length of useful thread||Length of vanish(or washout) thread|
So what can you do if threads don’t quite match up or you need to join multiple pipes or components?
Adapting Pipe Threads through Fittings
Threaded pipe fittings are likely your answer.
Like a threaded pipe, threaded fittings are available in a range of materials depending on your needs and budget.
Frequently used threaded pipe fittings include: