What Is NPT Thread? What Do the Abbreviations Mean?

NPT is a tapered pipe-thread standard commonly used on hydraulic fittings and other industrial connections. The threads are covered by ANSI/ASME B1.20.1 and tighten as the male and female threads are screwed together.

You may also see abbreviations such as NM and NF in some manufacturers’ part numbers. In those systems, NM can identify a male NPT connection, while NF can identify a female NPT connection.

How the NPT Taper Works

An NPT thread is not the same diameter from end to end. The thread gradually tapers, so the male and female threads become tighter against one another as the fitting is installed.

That mechanical interference helps create a secure connection, but the threads alone do not normally provide the entire pressure seal. Depending on the fitting and application, an approved thread sealant or PTFE tape may also be required.

Follow the fitting or equipment manufacturer’s instructions for the correct sealant and installation method.

If a connection starts threading together but quickly feels wrong, stop there. Different thread types can sometimes catch the first few threads even though they are not actually compatible.

NPT Is Not the Same as BSP

NPT and BSP fittings should not be treated as interchangeable.

The two systems can differ in thread form, pitch, taper, and the way the connection seals. A mismatched fitting may start into the port, but forcing it farther can damage both sides of the connection and leave you with a hydraulic leak.

Verify the thread type before tightening anything that does not feel right.

NPT threads are not interchangeable with BSP or other thread systems used on high- and medium-pressure fittings. Differences in thread form, pitch, taper, or sealing method can prevent the connection from sealing correctly. Mismatched fittings may appear to engage at first, but forcing them together can damage the threads and lead to hydraulic leaks. Verify the thread type and size before connecting hydraulic components.

Understanding NPT Sizes and Part Numbers

The numbers used in fitting descriptions can be confusing because they do not always represent a direct measurement of the thread itself.

Some manufacturers use numbers based on sixteenths of an inch in their part-number system.

For example, with a fitting identified as HNM4-NM2:

• The 4 represents 4/16″, or 1/4″.
• The 2 represents 2/16″, or 1/8″.
• That fitting would therefore be described as a 1/4″ NPT male × 1/8″ NPT male connection.

Not every manufacturer uses the same numbering method, so always check the actual fitting description before ordering.

Hi-Force, for example, uses model numbers such as HF17 for a 3/8″ NPT male × 3/8″ NPT male nipple.

At Winshaw, 3/8″ NPT and 1/4″ NPT fittings are commonly used across different hydraulic hose and tooling applications. Examples include 1/4″ male nipples such as the HF7, along with quick-connect couplings such as the CF1 and CM1.

How to Identify an NPT Thread

• Check the part number or manual first. If it is NPT, the thread type is often listed there.
• On a male fitting, the thread should narrow slightly toward the end instead of staying the same diameter all the way down.
• When the size is still unclear, use a thread gauge for pitch and calipers for diameter, then compare those numbers with the fitting specifications.
• Match those measurements to the fitting manufacturer’s specifications before ordering or installing a replacement.
• aIf the fitting still cannot be identified confidently, check with the equipment or fitting supplier before connecting it.

Practical Tips for Assembly

• Confirm both the nominal size and whether the connection is male or female.
• Start the fitting by hand so you can feel immediately if it is trying to cross-thread.
• Use the sealant specified for the fitting and hydraulic system.
• When PTFE tape is permitted, apply it in the direction that keeps it from unwinding as the fitting is tightened.
• If the fitting starts binding unusually early, stop instead of adding more force.
• For a connection that continues to leak, verify the thread type and condition before simply tightening it farther.

What Not to Mix With NPT

Do not assume two pipe fittings are compatible just because they look close in size.

NPT, BSP, and other hydraulic thread systems can be similar enough to cause confusion at the bench but different enough to damage a port once a wrench is applied.

When the thread type is unknown, identify it first. Replacing the correct fitting is much easier than repairing damaged threads in a hydraulic manifold, pump, or tool body.

Need Help Identifying a Fitting?

If you are unsure about an NPT size or need help matching a fitting to a Winshaw hydraulic application, contact the Winshaw team before forcing a questionable connection.

You can also learn more about Winshaw’s products and services in Winshaw – Your Partner in Construction.

For related hydraulic-system topics, see Hydraulic manifolds: How to choose a design and where to buy them and Hydraulic torque wrench tolerances — Accepted tolerances for hydraulic torque tools.