Plumbing and industrial settings use fittings to join pipes, hoses, and various pieces of machinery. These days National Pipe Thread (NPT), and British Standard Pipe (BSP) are the two most common thread kinds. Though sometimes similar, these two kinds of fittings have great differences and cannot substitute for one another. This guide provides a clear-cut technique for measurement.

What are NPT and BSP fittings?

NPT: National Pipe Thread

Most NPT fittings are utilized in the US and are governed by guidelines created by the American National Standards Institute (ANSI). The tapering of the threads is one of the main features of NPT fittings. Such a design means the thread gets narrower as its length progresses, thus allowing it to seal perfectly when connected to another fitting.

Key Points in NPT

• Tapered threads with a taper angle of 1°47′.
• The angle of the thread is 60◦.
• The taper provides a tighter seal while tightening the threads.
• Used in applications for which a leak-tight seal integrity is important.

Common Uses of NPT:

• In-Home plumbing systems
• Gas pipelines
• Oil and fuel systems
• Hydraulic devices

BSP: British Standard Pipe

BSP fittings are common in Europe, the UK, Australia, and in some parts of Asia’s has two categories, which are

• BSPT: British Standard Pipe Tapered
• BSPP: British Standard Pipe Parallel

BSPT threads are much like NPT threads in that they are tapered, and BSPP threads would be the same size all along their length – parallel.

Key points of BSP:

• The threads have a tapered design in BSPT and are parallel in BSPP.
• The thread angle measures 55 degrees, nearly identical to the 60-degree angle of NPT.
• BSPP threads often need a gasket or O-ring to seal, but BSPT threads can seal the threads fit together.

Common Uses of BSP:

• Water and gas systems
• Air compressors
• Industrial machinery
• Irrigation systems

Main Differences Between NPT and BSP Threads

Before measuring, you must know the difference between NPT and BSP threads.

Thread Angle:

• NPT angle is 60 degree
• BSP has a 55-degree angle

Thread Type:

• NPT is always tapered
• BSP can be tapered (BSPT) or straight (BSPP)

Pitch (Threads Per Inch):

• The thread size can be similar, but the pitch of the threads may be a little different.

Seal Type:

• NPT forms a seal by using the threads
• BSPT also forms a seal by using the threads, while BSPP requires a gasket or O-ring to make a seal

Tools You’ll Need to Measure NPT and BSP Fittings

The following tools will be required to measure NPT and BSP fittings accurately.

• Thread Gauge: A thread gauge determines the threads’ pitch.
• Digital caliper(preferably): A caliper can measure the outside diameter of threads with a reasonable degree of accuracy.
• Pipe Thread Identification Chart: This chart matches your measurement to a common thread size.
• Measuring Tape: Good for bigger parts where a caliper might not be enough.
• Protractor (Optional): This can be used to measure the thread angle; though it’s not commonly required, standardized charts are used.

Step-by-Step Instructions NPT and BSP Fittings Measurement

1. Identify the Thread Type

check the type of fitting: NPT or BSP. Usually, you can say by the country where it was made (for example, the United States usually uses NPT), but it’s more precise to check it with a thread gauge.

• How to Use a Thread Gauge:
• Select the gauge that seems to have teeth that correspond to your thread.
• Screw the gauge into the threads. If it comes out snug, then you have the correct pitch.

2. Measure the outer diameter (OD)

Use a caliper to measure the outer diameter of male threads or the inner diameter of female threads in both NPT and BSP fittings.

• Steps:
• Unlock the caliper jaws and position them over the threads.
• Measurements should be recorded in either millimeters or inches.
• Refer to the chart to match your measurement with a specific pipe size.

              Measuring inner diameter (for female fittings):

• For female fittings, measure the inner diameter of the fitting. This can also be checked against a thread chart to find the fitting size. 

3. Measure the Thread Pitch

Thread pitch is normally measured in threads per inch; the NPT has this dimension, while the BSP uses measurements in millimeters.

• How to Measure:
• Take the thread gauge and press it onto the threads. If it fits well, then that is the pitch.

4. Establish whether the thread is tapered or parallel

Check whether a thread is tapered, with a conical approach, or parallel.

• For NPT (tapered): measure the diameter in several different places along the thread. If it is different, it’s tapered.
• For BSP: Measure the same. If it is BSPT, it will be tapered; if it is BSPP, it will remain the same size.

5. Check Measurements Against Standard

Once you have OD, pitch, and taper details, check them against a standard thread chart. In this way, you are sure to get the correct fitting size.

Example: Measuring A 1/2-Inch NPT And 1/2- Inch BSP Fitting

Let’s see how to measure both kinds:

1/2″ NPT Fitting:

• Measure the outside diameter: approximately 0.84 inches (21.3mm).
• Check the thread pitch: 14 TPI.
• Check the diameter measurement at the base and end to verify the taper.

1/2″ BSP Fitting:

• Measure outside diameter: approximately 0.82 inches (21 mm).
• Check the pitch: 14 TPI.
• For BSPT, look for a taper; for BSPP, it is the same diameter.

Common errors and how to avoid them:

• Confusing BSP with NPT: They may look similar, but a wrong fit can lead to leaks—Retest with thread gauge.
• Missing the Taper: Sometimes, it’s hard to see the taper. To be specific, use a caliper and measure at several points.
• Ignoring the Thread Angle: Even if it appears the size is the same, remember that thread angles are different: 60° for NPT and 55° for BSP.

Helpful Tips for Measuring in the Field

• Keep a Thread Chart: It’s handy to identify sizes and pitches quickly.
• Organize Your Fittings: Label NPT and BSP fittings to avoid confusion.
• Use appropriate quality tools. Good calipers and gauges ensure quality measurements.

Conclusion

Measuring NPT and BSP fittings is not difficult, mainly if you know the differences and follow a few simple steps to measure and identify fittings. Always use the right tools, double-check your measurements, and compare them with a standard chart. Whether you are well-seasoned or were trained yesterday, correctly measuring fittings is the key to saving time and effort.