If you’ve tried to replace some plumbing hardware only to find that it looks like old piece but won’t fit, you’re not alone. This is one of those “obvious” things we all fall prey to as mechanics (seasoned or not), because it’s a classic “trap.” It’s not because the thread is defective; it’s usually because they uses different standard threads and it doesn’t match. A half inch isn’t a half inch.
Pipe thread types are separated into separate groups and mixing them will lead to broken days and stripped threads. Knowing the British Standard Pipe system are the first step towards eliminating the guesswork. This brings us to BSP threads: The specifics is broken out in the chart above, and it’s important to note that their geometry is basically different than what you may be familiar with in American NPT standards.
Understanding BSP Pipe Threads
The most important point here is the thread angle. While NPT employs a 60-degree angle (with flat roots), BSP employs a 55-degree angle (with rounded roots and crests). That’s called the Whitworth profile and while it might sound like just a few degrees on your protractor, in the mechanical world every degree counts.
When you attempt to force a 60-degree thread into a 55-degree hole, its tips won’t engage properly… They’ll actually bite into the flanks. You get a poor seal instantly and you’re immediately cross-threaded. It is a small thing. Sure, but it matters a lot.
So the angle is only one part of this puzzle, but more important you have to know there are two primary variations within the BSP family. The first is the taper version called BSPT, and the other is the parallel version called BSPP. That’s where most people screw up.
BSPP threads is parallel. This means the diameter does not change from end to end. So because they don’t taper, they can’t be sealed by themselves. The seal has to come from somewhere else. Typically, something is bonded onto the face of the fitting, like a bonded washer or something else like an O-ring. Think of it like jar lid that isn’t using the threads to seal, but rather the rubber gasket.
This is why you see BSPP used so much in high pressure hydraulics. Under extreme pressure you don’t want your threads taking the sealing load. You want them to engage in a neat parallel fashion and use another dedicated seal.
The BSPT (British Standard Pipe Taper), however, has a taper built into it that causes the threads themselves to wedge into each other when tightened. By doing so, they forms an effective metal-to-metal seal that doesn’t require extra gaskets if you use thread compound or tape correctly. According to the chart, the taper used for BSPT is one-in-sixteen which is the right combination to pull the threads tight enough to prevent leakage. This is what you’ll often find being used in gas and water lines where traditional plumbing techniques are practiced.
Know ahead of time whether you have a tapered or parallel tread system before you begin your taping or cutting and you’ll be fine. Attempting to mix the two types will lead to frustration or cause you to strip the parallel thread when tightening.
To further confuse things, there’s also a set of sizing conventions. According to the chart, sizes runs from G 1/8 all the way up to G 4. Each has a corresponding outside diameter and threads per inch.
Observe the change in pitch. As you move to the smaller sizes (such as G 1/8), the thread pitch gets finer. For the larger industrial sizes, it drops to a coarser pitch. And it isn’t random. Smaller pipes with finer threads have more threads per inch. That gives them greater resistance to vibration. It also means you can make more precise adjustments. Larger pipes with coarser threads is less likely to gall when installed but are easier to get together quickly. Often G 1/2 turns out to be the most popular residential water supply size, serving as a sort of middle ground in frequency of use.
Hours can be spent trying to make something work when all that’s needed is a minute of time to confirm what kind of thread it is on a job site. If you’re not sure, bring a thread gauge. It’ll save you from costly errors.
The angles and root radii are close enough that they can’t always be seen by the naked eye (don’t trust your eyesight). Feel for the thread with your finger if possible, otherwise look at the marking on the fitting. Tapered threads tend to indicate a slight bevel; parallel threads typically has a flat face.
The bottom line with BSP threads is they’re all about the geometry. Not remembering which diameter is what. But learning how and why the design works.
Why does the tapered Whitworth have a rounder root? It makes the thread stronger by relieving stress concentrations on the thread itself. Why does the Whitworth taper exist? To generate a wedge. Why does the parallel section exist at all? It exists to protect the thread from being used as a seal.
With that knowledge, your whole approach to making a connection changes from trying to cram in something that doesn’t fit to selecting the proper tool and actualy solving the problem. That’s not luck; a leak-proof joint isn’t accidental. It’s a product of applying the proper form to the right use.
