Size matters, especially in pneumatic systems, where everything must fit precisely for the system to perform at its best. Specifications for pipework differ across geographical areas, and knowing how to measure BSP fittings starts with where they're going to be used. Seals are also classified differently for different purposes: BSP fittings tend to be found in marine applications, while NPT fittings are more commonly used for oil and gas applications.
How To Measure BSP Fittings
British Standard Pipe fittings are recognised all over the world, but understanding how to measure them can be confusing.
To determine the correct size, you should consider these basic elements:
Firstly, you'll find BSP fittings offer two types of connection, with either a parallel (BSPP) or a tapered (BSPT) thread (see below). In the North American market there's also an NPT thread type, which is sometimes confused with BSPT.
BSPP and BSPT fittings can be accurately identified simply by measuring the diameter of the thread. For BSPP fittings you'll measure the outer diameter (OD), while for BSPT fittings you'll measure the inner diameter (ID) because the thread is tapered. This means that its OD will change down the length of the fitting, so it can't be used to make a fixed calculation.
For BSPT fittings, therefore, you'll need to take a calliper reading from inside the bore to get the ID, which is then usually referred to as the dash size. These sizes correspond to the ID measurement, in increments of 1/16" (0.0625 inches). For example, -6 dash size corresponds to a fitting ID of 6/16" (0.375 inches or 3/8").
The thread pitch refers to the distance between adjacent threads, measured at their crest. Since the threads can be set at an angle, you have to measure them parallel to the axis of the thread, whereas the OD is measured at 90º to the thread axis.
To confirm that you do actually have a BSP fitting, count the number of threads on the fitting per inch. You can do this most easily by taking a specified length, such as ½" (0.5 inches), and multiplying it by 2. So if you count 6 thread crests over ½", that means you have 12 threads per inch. You can even halve this down to measuring threads over ¼" (0.25 inches) and multiplying the resulting figure by four.
Once you've measured the OD, you then subtract 25% from this figure to obtain the thread size. For example, if a BSPP male thread's OD measures 1" (one inch), subtracting 25% (0.25 inches) from that will give you a thread size of ¾" (0.75 inches).
All of these measurements can then be cross-referenced on the chart below to ascertain the standard BSP fitting size and its associated specifications.
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BSPT & BSPP
SIZE & PITCH
British Standard Pipe Fittings
British Standard Pipe fittings are divided into two categories:
1. British Standard Parallel Pipe (BSPP)
This type is popular throughout the UK and Europe, Australia, New Zealand, South Africa and Asia. BSPP connectors have a parallel thread, and the male connector is usually engineered with a 30° bevel so that it slots into the female thread's 30° recessed cone. Some BSPP fittings also need an O-ring or bonded seal washer to seal the connection.
2. British Standard Pipe Tapered (BSPT)
These connections are tapered at a 55° angle, and you can confirm this measurement with a thread gauge. Tightening the tapering threads are what seals the BSPT pipe fittings, but you'll need to use thread sealant to ensure that the two parts are sealed securely. It's not an ideal system, because insufficient lubrication or over-tightening the joint can easily damage the threads, especially in stainless steel fittings. It is possible to fit male BSPT threads into female BSPP connectors, but it's not a particularly good idea, because you won't get a proper seal.
NPT (National Pipe Thread) Fittings
The NPT fittings found in Canada and the US are similar to BSPT fittings, except that NPT threads are tapered at a 60° angle rather than 55°. They also tend to have a different thread pitch to BSPT pipe sizes, but you can sometimes fit a male NPT connector into a BSPT female and a male BSPT connector into an NPT female.