BSP (Most common pipe thread in NZ)
BSP thread form stands for British Standard Pipe and is common in New Zealand and the commonwealth countries. It is based on "trade" size rather than actual diameter which can lead to some confusion when measuring fittings.
There are two types of BSP threads;
- BSPP - Female thread are both (also known as G)BSPT
- The female thread is parallel and the male thread is tapered (also called R/Rp) (the female thread can also be tapered it is then Rc, (these are fairly rare ).
- They typically come in trade sizes which are related to schedule pipe sizes (in inches) or the relative nominal "Trade bore" size of the pipe in metric (mm) 1/8 (4mm), 1/4 (6mm), 3/8 (10mm), 1/2 (15mm), 3/4 (20mm), 1 (25mm), 1-1/4 (32mm), 1-1/2 (40mm), 2 (50mm), 2-1/2 (65mm), 3 (80mm), 4 (100mm).
- For example, someone might say in trade language "100mm BSP thread" However there is no exact 100mm dimension on the threaded fitting, this is also known as a 4" fitting and the thread would most likely be Tapered.
When using BSPT, it is also common to call the female thread BSPP (parallel) and the male BSPT (tapered) even though they are both technically a BSPT thread form (the female would be parallel and the male would be tapered).
Both threads have the same pitch, angle (55 degrees) and shape (rounded peaks and valleys).
The chart gives the major and minor diameters for each BSP Trade Thread Size. The minor diameter may vary from what is listed depending upon where it has been manufactured to in the thread gauge. Unfortunately, nowadays these variances are common can be challenging, especially with materials such as stainless and plastics.
NPT (Most Common pipe thread in the USA and US-made equipment)
"NPT (National Pipe Thread) is a US standard thread type. It's also known as:
- MPT/MNPT/NPT(M) for male threads
- FPT/FNPT/NPT(F) for female threads
Tips:
- A thread sealant is required for a leak-free seal (except for NPTF threads)
- NPT is based on trade size, not actual diameter (similar to BSP)
- Thread characteristics:
- Pitch: same for all NPT threads
- Angle: 60 degrees with flat peaks and valleys
The table below provides Threads Per Inch, Pitch, and Major Diameter for NPT Threads.
Hack: Although not recommended and designed to, small BSP and NPT threads in brass and plastic will screw into each other up to about 1/2 (15mm) Trade size as the pitch and TPI is very similar on low-pressure fluids and vapours with suitable thread sealant they actually seal up fine. Stainless is often machined with more precision and will bind up.
How To Measure Threads
To identify the trade size of the thread, first figure out whether the thread is tapered or parallel. This is done by measuring the first, fourth/fifth and last full thread. If the diameter increases from first thread to the last thread on a male thread or decreases from first to last thread on a female thread, then the thread is classified as a tapered thread (such as BSPT). If the diameter stays the same it's parallel thread (BSPP). Put them both side by side and see if you can spot the difference.
Once it is known whether the thread is parallel or tapered, it can be measured to determine it’s trade size. This trade size will not directly represent what is measured refer to the chart.
For parallel threads, any thread can be used to measure for its trade size, and for a tapered thread the fourth or fifth full thread is used. This measurement can then be referenced against a thread table to match it to its trade size in the chart.
Dimensions may differ slightly between measurement and dimensions of the trade sizes because of variances in tolerances in measuring and manufacturing.
