Straight thread is applied in pipes, tubes, and fittings in various industries, understanding NPS threads can ensure the connections are made accurately and the part or system can function properly. In this article, we’ll explain the straight pipe thread and NPS thread, NPS and NPT differences, and focus on NPSM/NPSL thread dimensions charts.
What Is Straight Pipe Thread?
Straight pipe thread, also known as parallel thread or non-tapered thread, is a type of thread used to connect pipes or tubes in a piping system. Unlike tapered threads, which are commonly used in pipe fittings, straight pipe threads have a constant diameter and pitch throughout the entire length of the thread. The thread is parallel to the axis of the pipe, thread flanks are parallel to each other. The pipe of the thread, which is the distance between two consecutive threads, remains the same throughout the length.
What Is NPS Thread?
NPS stands for National Pipe Straight, NPS thread is a type of straight thread used to connect pipes and fittings in a piping system.
What is the NPS thread used for?
– Oil and gas: pipelines, refineries, and processing facilities.
– Water and wastewater: water treatment plants, wastewater treatment plants, and distribution systems.
– Chemical processing: chemical plants, refineries, and other processing facilities.
– Power generation: power plants, including fossil fuel, nuclear, and renewable energy facilities.
Types of NPS Threads
1. NPSM (National Pipe Straight Mechanical) threads: used for mechanical joints in power generation industries and more.
2. NPSC (National Pipe Straight Coupling) threads: used for coupling in oil and gas and more.
3. NPSL (National Pipe Straight Locknut) threads: used for locknuts in chemical processing and more.
4. NPSF (National Pipe Straight Fuel) threads: used for fuel systems, including aircraft and automotive applications.
5. NPSH (National Pipe Straight Hose) threads: used for hose connections in hydraulic, pneumatic, and fuel systems.
6. NPSI (National Pipe Straight Instrument) threads: used for instrument connections, including gauge, sensor, and transmitter connections.
7. NPSO (National Pipe Straight O-Ring) threads: used for O-ring connections.
NPS vs NPT Pipe Thread, What Are the Differences?
NPS (National Pipe Straight) and NPT (National Pipe Taper) are both thread standards used in piping systems, but there are some differences between them.
– Thread shape: NPS threads are straight, while NPT threads are tapered.
– Taper: NPS threads have no taper, which means the thread diameter remains the same throughout its length, while NPT threads have a taper, and the thread diameter decreases as the thread progresses.
– Pitch: NPS threads have a fixed pitch, while NPT threads have a variable pitch.
– Sealing: NPT threads are designed for a tighter seal than NPS threads.
– Applications: NPS threads are commonly used in high-pressure applications, such as in oil and gas, chemical processing, and power generation industries. while NPT threads are commonly used in applications requiring a tight seal, such as in hydraulic, pneumatic, and gas systems.
NPS and NPT pipe thread, which is better? When high-pressure ratings are required or a straight thread is necessary for ease of assembly and disassembly, use NPS, when a tight seal or secure connection is needed, NPT is recommended.
NPS Thread Chart (Straight Pipe Thread Chart)
NPS threads are available in various sizes, ranging from 1/8 to 12 inches in diameter, below we have four tables for showing the NPSM and NPSL thread dimensions for both external and internal, the unit used in the chart is the inch. You can check out some common sizes such as 1/2-14 NPSM thread dimensions.
NPSM Thread Dimensions Chart (External Thread)
| Nominal Pipe Size |
Threads per Inch |
Allowance | Major Diameter | Pitch Diameter | ||
| Max. aa | Min. | Max. | Min. | |||
|
1/8
|
27
|
0.0011
|
0.397
|
0.390
|
0.3725
|
0.3689
|
|
1/4
|
18
|
0.0013
|
0.526
|
0.517
|
0.4903
|
0.4859
|
|
3/8
|
18
|
0.0014
|
0.662
|
0.653
|
0.6256
|
0.6211
|
|
1/2
|
14
|
0.0015
|
0.823
|
0.813
|
0.7769
|
0.7718
|
|
3/4
|
14
|
0.0016
|
1.034
|
1.024
|
0.9873
|
0.9820
|
|
1
|
11 1/2
|
0.0017
|
1.293
|
1.281
|
1.2369
|
1.2311
|
|
1 1/4
|
11 1/2
|
0.0018
|
1.638
|
1.626
|
1.5816
|
1.5756
|
|
1 1/2
|
11 1/2
|
0.0018
|
1.877
|
1.865
|
1.8205
|
1.8144
|
|
2
|
11 1/2
|
0.0019
|
2.351
|
2.339
|
2.2944
|
2.2882
|
|
2 1/2
|
8
|
0.0022
|
2.841
|
2.826
|
2.7600
|
2.7526
|
|
3
|
8
|
0.0023
|
3.467
|
3.452
|
3.3862
|
3.3786
|
|
3 1/2
|
8
|
0.0023
|
3.968
|
3.953
|
3.8865
|
3.8788
|
|
4
|
8
|
0.0023
|
4.466
|
4.451
|
4.3848
|
4.3771
|
|
5
|
8
|
0.0024
|
5.528
|
5.513
|
5.4469
|
5.4390
|
|
6
|
8
|
0.0024
|
6.585
|
6.570
|
6.5036
|
6.4955
|
NPSM Thread Dimensions Chart (Internal Thread)
| Nominal Pipe Size |
Threads per Inch |
Minor Diameter
|
Pitch Diameter
|
||
|
Min. aa
|
Max.
|
Min.
|
Max.
|
||
|
1/8
|
27
|
0.358
|
0.364
|
0.3736
|
0.3783
|
|
1/4
|
18
|
0.468
|
0.481
|
0.4916
|
0.4974
|
|
3/8
|
18
|
0.603
|
0.612
|
0.6270
|
0.6329
|
|
1/2
|
14
|
0.747
|
0.759
|
0.7784
|
0.7851
|
|
3/4
|
14
|
0.958
|
0.970
|
0.9889
|
0.9958
|
|
1
|
11 1/2
|
1.201
|
1.211
|
1.2386
|
1.2462
|
|
1 1/4
|
11 1/2
|
1.546
|
1.555
|
1.5834
|
1.5912
|
|
1 1/2
|
11 1/2
|
1.785
|
1.794
|
1.8223
|
1.8302
|
|
2
|
11 1/2
|
2.259
|
2.268
|
2.2963
|
2.3044
|
|
2 1/2
|
8
|
2.708
|
2.727
|
2.7622
|
2.7720
|
|
3
|
8
|
3.334
|
3.353
|
3.3885
|
3.3984
|
|
3 1/2
|
8
|
3.835
|
3.848
|
3.8888
|
3.8988
|
|
4
|
8
|
4.333
|
4.346
|
4.3871
|
4.3971
|
|
5
|
8
|
5.395
|
5.408
|
5.4493
|
5.4598
|
|
6
|
8
|
6.452
|
6.464
|
6.5060
|
6.5165
|
NPSL Thread Dimensions (External Thread)
| Nominal Pipe Size |
Threads per Inch |
Major Diameter (Max) | Pitch Diameter (Max) | Pitch Diameter(Min) |
|
1/8
|
27
|
0.409
|
0.3840
|
0.3805
|
|
1/4
|
18
|
0.541
|
0.5038
|
0.4986
|
|
3/8
|
18
|
0.678
|
0.6409
|
0.6357
|
|
1/2
|
14
|
0.844
|
0.7963
|
0.7896
|
|
3/4
|
14
|
1.054
|
1.0067
|
1.0000
|
|
1
|
11 1/2
|
1.318
|
1.2604
|
1.2523
|
|
1 1/4
|
11 1/2
|
1.663
|
1.6051
|
1.5970
|
|
1 1/2
|
11 1/2
|
1.902
|
1.8441
|
1.8360
|
|
2
|
11 1/2
|
2.376
|
2.3180
|
2.3099
|
|
2 1/2
|
8
|
2.877
|
2.7934
|
2.7817
|
|
3
|
8
|
3.503
|
3.4198
|
3.4081
|
|
3 1/2
|
8
|
4.003
|
3.9201
|
3.9084
|
|
4
|
8
|
4.502
|
4.4184
|
4.4067
|
|
5
|
8
|
5.564
|
5.4805
|
5.4688
|
|
6
|
8
|
6.620
|
6.5372
|
6.5255
|
|
8
|
8
|
8.615
|
8.5313
|
8.5196
|
|
10
|
8
|
10.735
|
10.6522
|
10.6405
|
|
12
|
8
|
12.732
|
12.6491
|
12.6374
|
NPSL Thread Dimensions Chart (Internal Thread)
| Nominal Pipe Size |
Threads per Inch |
Minor Diameter (Min) | Pitch Diameter (Min) | Pitch Diameter (Max) |
|
1/8
|
27
|
0.362
|
0.3863
|
0.3898
|
|
1/4
|
18
|
0.470
|
0.5073
|
0.5125
|
|
3/8
|
18
|
0.607
|
0.6444
|
0.6496
|
|
1/2
|
14
|
0.753
|
0.8008
|
0.8075
|
|
3/4
|
14
|
0.964
|
1.0112
|
1.0179
|
|
1
|
11 1/2
|
1.208
|
1.2658
|
1.2739
|
|
1 1/4
|
11 1/2
|
1.553
|
1.6106
|
1.6187
|
|
1 1/2
|
11 1/2
|
1.792
|
1.8495
|
1.8576
|
|
2
|
11 1/2
|
2.265
|
2.3234
|
2.3315
|
|
2 1/2
|
8
|
2.718
|
2.8012
|
2.8129
|
|
3
|
8
|
3.344
|
3.4276
|
3.4393
|
|
3 1/2
|
8
|
3.845
|
3.9279
|
3.9396
|
|
4
|
8
|
4.343
|
4.4262
|
4.4379
|
|
5
|
8
|
5.405
|
5.4884
|
5.5001
|
|
6
|
8
|
6.462
|
6.5450
|
6.5567
|
|
8
|
8
|
8.456
|
8.5391
|
8.5508
|
|
10
|
8
|
10.577
|
10.6600
|
10.6717
|
|
12
|
8
|
12.574
|
12.6569
|
12.6686
|
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