When it comes to hose and tube fittings, there are a number of different types of threads available. The number of fitting end options can make selecting the most appropriate fitting thread type a daunting task. The reliability of the fitting connection depends on selecting what’s optimal for the application. First, we will educate you about six types of common thread types. Second, we will guide you through the thread identification process.
Six Most Common Types of Threads
When it comes to different thread types, most people who work in the fluid-power industry will recognize American pipe threads (NPT/NPTF) as well as SAE or Unified threads (UN/UNF). However, threads and connections are divided into six main types:
- BSPP (BSP, parallel)
- BSPT (BSP, tapered)
- metric parallel
- metric tapered
Let’s learn the difference between each one of them.
How to identify different types of
threads in four easy steps
Before you start, make sure you have two thread identifying tools on hand:
- Pitch Gauge
A pitch gauge is a tool used to measure the distance between the crests of threads. For NPT, UN/UNF, BSPP, and BSPP we measure the number of threads per inch. If you have the metric threads, the pitch gauge identifies the distance between each individual crest in millimeters.
A caliper is used to measure the diameter of a thread. For male threads, it measures the outside diameter, for a female thread – inside diameter, respectively. For advanced users, a digital caliper is available from multiple retailers. It saves your time and simplifies the process.
STEP 1. Parallel thread vs. tapered thread. Let’s learn the difference.
Parallel threads include:
- metric parallel
Tapered threads include:
- metric tapered
To identify whether the thread is tapered or parallel, look at the diameter of your thread. If the thread diameter gets thinner towards the end, you are looking at a tapered thread. On the contrary, if the thread diameter is the same at the top and the bottom of the thread, you have the parallel thread. See the graphics below to learn the visual difference between tapered and parallel threads. Using a known parallel, like a pair of calipers can help to show if a fitting is tapered. If the threads touch the entire length of the calipers, it is parallel but if it rocks you have tapered threads.
STEP 2. Determine the Pitch
Use a thread pitch gauge tool to determine the size of the thread. Thread pitch gauge helps you to accurately measure and calculate the number of threads within a given distance.
Try a number of gages from the pitch gauge tool against the white background before deciding which one fits your thread the best. Look at the common pitch sizes for each of the different thread types in the table below.
|Thread Type||Pitch Size|
|12, 14, 16, 18, 20, 24|
|11 ½, 14, 18, 27|
|11, 14, 19, 28|
|11, 14, 19, 28|
|Metric Parallel||1.0, 1.5, 2.0|
|Metric Tapered||1.0, 1.5, 2.0|
STEP 3. Thread size matters
When it comes to determining the thread size, you have two approaches at your disposal. Before you start, determine if your thread is a pipe thread (NPT/NPTF, BSPT, BSPP) or not (UN/UNF, Metric Parallel, Metric Tapered). Remember that tapered thread can both be a pipe thread or not.
If you have a pipe thread, compare the size of the thread with a nominal size profile, as shown in Figure 1. For non-pipe thread (UN/UNF, Metric Parallel, Metric Tapered), use the caliper tool to measure the outside diameter of the tread.
STEP 4. Designate the thread
This step involves designating the thread according to the industry standards for further use. Start with identifying the tread size (nominal or actual), then write down the type and the pitch (if applies). See the examples in the table below under STEP 4:
|Thread Type||Step 1. Tapered or parallel||Step 2. Determine the pitch||Step 3. Thread size matters||Step 4. Designate the thread|
|UN/UNF (SAE)||Parallel||12, 14, 16, 18, 20, 24||Measure with caliper||Size-pitch, type 3/4-16 UN/UNF|
|NPT/NPTF||Tapered||11 ½, 14, 18, 27||Compare with profile||Size-pitch, type 1/4-18 NPT|
|BSPP||Parallel||11, 14, 19, 28||Compare with profile||G, size* G1/8|
|BSPT||Tapered||11, 14, 19, 28||Compare with profile||R, size* R1/2|
|Metric Parallel||Parallel||1.0, 1.5, 2.0||Measure with caliper||M, size X pitch M14X1.5|
|Metric Tapered||Tapered||1.0, 1.5, 2.0||Measure with caliper||M, size x pitch, keg or Taper, M10 x 1 keg or Taper|
To sum up, the thread ID process consists of four simple steps. We have learned about different types of threads, including tapered and parallel, pitch sizes, thread sizes, and industry standards when it comes to identifying thread types. Remember that selecting the wrong part can result in physical or property damage if resulting in a leak, so always consult you, fluid connector specialist, if you have any questions to prevent your equipment from breaking or malfunctioning.
Pipe Thread Acronyms:
|BSP||British Standard Pipe|
|BSPP||British Standard Pipe Parallel Thread|
|BSPT||British Standard Pipe Taper Thread|
|NPT||National Pipe Thread|
|NPTF||National Pipe Thread Fuel|
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