Male or Female
Thread gender can be determined by observing whether the threads are located internally or externally. Threads located externally are Male and threads located internally are Female. In some cases, the gender of the thread is included in part descriptions e.g. MPT (Male Pipe Thread) or FPT (Female Pipe Thread).
Parallel or Tapered
Determining the difference between parallel and tapered thread is also a straight forward process. If the outer diameter of the thread on a fitting remains constant, the thread type is Parallel. Another simple way to identify Parallel threads are by its use of an O-ring or gasket. The O-ring or gasket is a requirement due to parallel threads being unable to create a strong seal.
Tapered threads can be identified by its conical shape i.e. the thread diameter changes by narrowing towards the base of the thread body. This taper helps create a seal due to the wedge effect. It is always recommended to use thread sealant on tapered threads to ensure proper seal is made.
Common Thread Types
Reference below for common thread types offered on SPC fittings.
NPT – National Pipe Thread is an American standard most commonly used in the US and Canada.
BSPT – British Standard Pipe Taper Thread is very similar to NPT thread and sometimes referred to as R thread. It is found most commonly on equipment made outside of the US such as Europe or Asia.
If thread is TAPERED AND
If your equipment uses INCH tubing, it will most likely use NPT thread.
If your equipment uses METRIC tubing, it will most likely use BSPT thread.
BSPP – British Standard Pipe Parallel Thread also commonly referred to as G thread is commonly found on equipment made outside of the US. One easy indicator aside from the parallel thread would be the use of an O-Ring or gasket.
Metric – ISO Metric Screw Thread is found most commonly on equipment made outside of the US such as Europe or Asia. Similar to BSPP thread, it will utilize an O-ring or gasket. Available thread sizes tend to be quite small ranging from 3mm to 8mm.
UNF – Unified National Fine Thread is the American analog to Metric Screw thread. With very similar characteristics, UNF thread sizes opt for diameters that are some fraction of the inch measurement system. Again, O-ring or gasket is utilized.
If thread is PARALLELAND
If your equipment uses METRIC tubing, it will most likely use BSPP thread.
If your equipment uses METRIC tubing and thread size is under 8mm, it will most likely use Metric thread.
If your equipmet uses INCH tubing and thread size is under 8mm, it will most likely use UNF thread.
Pitch and Major Outer Diameter
This step is perhaps the most challenging and the cause of many mistaken orders due to the difficulty of getting proper measurements without the appropriate tools.
With a good pitch gauge and caliper, measurements can be taken easily and compared to a thread identification chart to determine the correct thread size. In the absence of proper measuring tools, the following descriptions and charts should help.
Pitch is the distance measured between threads. US or British (Inch) threads will use a slightly different method to calculate pitch vs Metric threads. Inch threads count the total number of Threads Per Inch called TPI. Metric threads simply measure the distance between peak to peak of neighboring threads called TP or Thread pitch.
Major Outer Diameter is another important measurement to determine thread size. WARNING: For US and British pipe threads (NPT, BSPT, BSPP), the major outer diameter is NOT equal to thread size. A common mistake when a user measures major diameter is to order the same thread size e.g. Major diameter measures close to 0.5″ so a purchase is made for 1/2″ NPT thread. As you can see from the thread identification chart below, what was really needed was a 1/4″ NPT. For metric threads, things are more intuitive, and the measured outer diameter IS equal to thread size e.g. Major diameter measures 10mm, then thread size is M10.
Major Outer Diameter: Measure 3-4 threads below widest point
Thread Size and
Thread Identification Chart
Once you have made Pitch and Major Diameter measurements, you can use these values to find the associated Thread Size by referencing a Thread Identification Chart. As you can see below, the Major Diameter is not the same as the Thread size for US and British pipe thread. It may be difficult to differentiate between NPT and BSPT because the major diameters and pitch are so similar. In this case, please refer to the General Heuristics in Step 2.
Note: This chart is for available thread sizes on SPC fittings. More extensive charts can easily be found online.