Pressure Gauge Connection Types and Thread Sizes: BSPP, BSPT and NPT

Pressure gauge connections in Australia fall into two main families: British Standard Pipe threads (BSP, with parallel BSPP and tapered BSPT versions) and American National Pipe Taper threads (NPT). They look very similar but are not interchangeable, because BSP threads have a 55 degree thread form and NPT threads have a 60 degree thread form. Most industrial gauges use a 1/4 inch or 1/2 inch connection, and the position of the port on the case, such as lower mount or back mount, matters just as much as the thread. This guide explains each thread type, how to identify an existing port, how sealing works and how to avoid the most common ordering mistakes.

Why connection types matter

A gauge that does not fit the port is a wasted order and a delay on site. Worse, forcing mismatched threads together can damage the port, cause leaks or let the gauge blow out under pressure. Because Australian plants contain equipment from Europe, Asia and North America, you will often find several thread standards on the same site. Confirming the thread before you order is the single most effective way to avoid problems.

The main thread types

BSPP (G thread): British Standard Pipe Parallel

BSPP is a parallel, or straight, thread. It is often written as G followed by the size, for example G1/4. Because the thread does not taper, it cannot seal on the thread alone. The seal comes from a flat face, a bonded washer (sometimes called a Dowty seal) or an O-ring at the base. The PDK PDR100, PDR250 and PDR500 digital gauges use a G1/4″ BSPP male bottom-entry connection.

BSPT (R thread): British Standard Pipe Taper

BSPT is a tapered thread, often shown as R or R1/4 on the male part. The thread tightens as it is screwed in and seals on the threads, with sealing tape or paste filling any small gaps. Our L63 liquid filled gauge uses a 1/4″ BSPT connection, and the L100 uses a 3/8″ BSPT connection. The KPCh low pressure gauge also uses 1/4″ BSPT.

NPT: National Pipe Taper

NPT is the standard American tapered pipe thread. It seals on the threads with a sealant, like BSPT, but the thread form differs. Several of our gauges use NPT: the AD-D-2 has a 1/4″ NPT lower mount, and the AL-F-2, AL-H-2 and Solfrunt 1981 gauges have a 1/2″ NPT lower mount. The Baroli BD Series digital gauge is available with 1/4″ BSP or NPT and 1/2″ NPT connections.

Other fittings on test equipment

Hydraulic test points and hoses also use unified (UNO) threads and quick-connect fittings. The PDR Digital Pressure Test Kit includes adaptors for several of these so one gauge can connect to many ports.

BSP and NPT are not interchangeable

This is the most common mistake. A 1/4 inch BSP thread and a 1/4 inch NPT thread may start together in your fingers, and then jam or leak. The key differences are:

The similar thread counts mean the 1/2 inch sizes in particular can be mistaken for each other. Always identify the thread form with a gauge or a thread pitch tool before you buy.

Common sizes and their approximate dimensions

The nominal size, such as 1/4 inch, refers to the pipe bore, not the thread diameter. Typical outside diameters of the male thread are:

• 1/8 inch: about 9.7 mm for BSP and 10.3 mm for NPT
• 1/4 inch: about 13.2 mm for BSP and 13.7 mm for NPT
• 3/8 inch: about 16.7 mm for BSP and 17.1 mm for NPT
• 1/2 inch: about 21.0 mm for BSP and 21.3 mm for NPT

These measurements help you recognise a thread with callipers, but a thread pitch gauge is a better test, because the diameters are so close.

Port position: lower mount, back mount and centre back

The thread is only half the story. Where the connection leaves the case changes how the gauge sits.

• Lower mount (bottom connection): the connection points straight down. This is the standard for pipe-mounted gauges. Our L63, L100, AD-D-2, AL-F-2, AL-H-2 and Solfrunt 1981 are all lower mount.
• Back mount (rear connection): the connection comes out the back of the case. It is used for panel and flush mounting. The L63 and L100 can be supplied with a back connection.
• Centre back mount (CBM): the connection is in the middle of the back, often for panels. The KPCh low pressure gauge is available as a lower mount (LM) or a centre back mount (CBM).

Switching from lower to back mount means changing the pipework or the panel cut-out, so it should be a deliberate decision. If you need a gauge in a panel, check the option for a front flange or a panel mount kit. Our datasheets list these options for several models.

How to seal each connection

Tapered threads (BSPT and NPT)

1. Clean the male and female threads and check that they are undamaged.
2. Apply PTFE tape in the direction of the thread, or a suitable pipe sealant, leaving the first thread bare.
3. Screw the gauge in by hand, then tighten with a spanner on the flats of the gauge socket, not on the case.
4. Stop when the gauge is firm and the dial faces the right way. Over-tightening can crack the port or distort the tube.

Parallel threads (BSPP)

1. Fit a bonded washer, O-ring or other seal under the shoulder of the thread.
2. Screw in by hand, then tighten to the recommended torque.
3. Do not use tape to seal a parallel thread. It will not seal and can contaminate the system.

Never use the case to turn the gauge. Use a spanner on the socket flats only. This protects the movement and the accuracy of the gauge.

Adaptors and what to watch for

If you cannot change the port, an adaptor can bridge two thread types. Use quality adaptors rated for the full pressure, and keep the number of joints to a minimum, because each one is a potential leak. Check that the adaptor material suits the media and that it does not add a restriction that damps the pressure signal.

For permanent installations, consider an isolation valve or a needle valve between the process and the gauge. That allows you to remove the gauge for calibration without shutting down the line. A snubber or restrictor can also reduce wear on the gauge in a pulsating line. Some of our gauges accept optional restrictors.

How to identify an unknown thread

1. Measure the outside diameter of the male thread with callipers, and compare with the list above.
2. Count the threads per inch with a thread pitch gauge.
3. Check the thread angle. A 55 degree gauge fits BSP, and a 60 degree gauge fits NPT.
4. Look for a taper. If the thread narrows toward the end, it is tapered (BSPT or NPT). If the diameter is constant, it is parallel.
5. Check for a seal face or washer groove, which suggests BSPP.
6. When in doubt, photograph the thread and send it to us.

Connection material

The connection material should match the media and the environment. Brass and copper alloy are common on air, water and oil. Stainless steel is the choice for corrosive media. The L63 and L100 use a copper alloy connection, the AD-D-2, AL-F-2 and AL-H-2 use 316L stainless steel, and the KPCh uses a brass connection. The Baroli BD Series offers 316Ti or 316L stainless steel or an alumina ceramic option. Our guide to liquid filled vs dry pressure gauges covers how the choice of material interacts with the case fill.

Quick reference before you order

Write down four things: the thread family (BSPP, BSPT or NPT), the nominal size (1/8, 1/4, 3/8 or 1/2 inch), the port position (lower mount, back mount or centre back mount) and the seal type on the existing port. If you can supply a clear photo of the old gauge and its thread, our team can confirm a match quickly. For digital gauges, also check whether the port is bottom entry, as it is on the PDK PDR range.

Frequently asked questions

Can I screw a BSP gauge into an NPT port?

You should not. The thread angles differ, so the threads will not seat properly and may leak or damage the port. Use the correct thread or a purpose-made adaptor.

Is G1/4 the same as 1/4 BSPP?

Yes. G is the ISO designation for a parallel pipe thread, and G1/4 is the same as 1/4 inch BSPP.

What does “lower mount” mean?

It means the connection exits at the bottom of the case, in line with the vertical axis of the gauge. It is also called a bottom connection.

Do digital gauges use the same threads?

Yes. They use the same BSP and NPT thread families, so the points above apply. Most of our PDK digital gauges use G1/4 BSPP, so check your port carefully.

How tight should I make the connection?

Firm, but not excessive. Follow the manufacturer’s torque guidance where given, and use a spanner on the socket flats only.

Next steps

Once you know the thread, choose the range with our pressure gauge range selection guide, then explore the pressure gauges and digital pressure gauges in our range. If you are unsure about a thread, contact us with a photo and the dimensions, and we will help you match it.