In a hardware aisle, you pick up two similar-looking fittings: same material (metal), same type of thread (external), but two different countries (one from Ohio, one from Germany). Screw them together and … water leaks. Why? Because one fitting has an American NPT thread; the other, British Standard Pipe. The geometry is different; you can’t apply torque until you know what you’re dealing with. A common error. Here are the visual references that clarify the physical differences to help you avoid failure.
First off you see the thread angle. BSP threads has rounded roots and crests, while NPT threads have sharp flats. They also have a fifty-five degree cut instead of sixty degrees. That’s the geometry that determines the seal.
Why NPT and BSP Fittings Are Different
Then there are the tapered versions where both standards taper at a one in sixteen ratio, but each standard engage the taper differently. Adding more tape doesn’t make it compatible any more than cranking down harder. The mating surfaces will never mate correct and the threads will simply cross and strip. It’s not an issue of being mislabeled but rather a physical difference.
The type of BSP threads you use will determine how you seal it. Standard BSP is parallel (same dia across the whole length), and don’t actualy seal against the other BSP. They rely on a face seal, where you insert some sort of washer (bonded, or O- ring) between the flat faces and crank down on it until rubber compresses into a seal. The tapering BSP threads act more like good old American pipe: As you twist them together, they wedge tighter and tighter. Here, you want to use a sealer compound, or PTFE tape, on the ridge itself. Tape won’t help at all on a parallel fitting because there is no compression along the sides of the thread. That compression is what prevents most leaks from occurring in the first place.
This leads into size identification. Again, be careful on this one too. What size is a 1/2″ BSP fitting? The answer isn’t what you think. Because it’s measured from inside the bore of the original pipe they was made to thread into hundreds of years ago, a 1/2″ BSP fitting has an outer diameter that’s more like twenty-one millimeters. This means a 1/2″ BSP fitting is larger than a 1/2″ piece of pipe in any other system. You can follow the chart above. It shows how the sizes increase as you step up in size and how the pressure ratings decreases greatly when the diameter gets bigger. This is because smaller threads can handle higher pressures. They have less surface area for the pressure to act upon, and the walls are thicker relative to the bore. If you don’t take this change in stress distribution into account when running your high pressure hydraulics through oversized fittings, it’ll only invite failure.
That’s not made any easier when traveling around the world because almost all equipment manufactured in Asia or Europe will be BSP. Because it originated from the UK, that makes sense; they colonized and then sold their products everywhere, so the standard traveled as well. However, North America bucks that trend with NPT or NPS systems. That’s the major exception where different regions mean trouble if you mix them together without an adapter. Within a single country, even, where equipment is imported, one might get both. It’s safer still just to double-check the angle with a protractor or compare to a known good fitting rather than assuming anything based off the label.
While there is a ton of variation out there, basic principles remain unchanged. Whatever the shape of your elbow, tee, union, etc., they will all be threaded in the same way. The size reducer adjusts the diameter while retaining the thread standard through the piece. Just like before, one end may taper and the other may be parallel. To make matters more confusing, many fittings incorporates both ends so that two dissimilar system can be connected. If you aren’t careful when you’re dealing with these and paying close attention to which end you’re using, you’ll soon be spinning in circles.
In conclusion. The geometry matches the seal. Respect the face and the angle. Don’t take the outside diameter for granted; use a tape measure instead. Check the taper before you commit to turning it. Avoiding the drip now will save hours of frustration once system is dry and holding no pressure.
