Pipe threads vary depending on national and international standards and the field of application. This article describes the differences and compatibilities between the BSPP (or gas), BSPT, and NPT threads.
Pipe thread types
There are two basic types of the pipe thread:
- Parallel (also called straight) — the diameter of the pipe thread is constant.
- Tapered — the diameter of the pipe thread is not constant. It increases or decreases as the thread length advances.
There are two most common pipe thread standards:
- British Standard Pipe Thread (BSP) is based on a 55-degree angle thread. This standard has been adopted worldwide (except in North America).
- American National Standard Pipe Thread is based on a 60-degree angle thread. This standard is the most widely used in the United States and Canada.
Also, each pipe thread standard features a specific number of threads per inch and different shapes of thread peaks and valleys.
BSPP thread
BSPP (or gas) thread is a parallel thread of the BSP standard (the P at the end stands for Parallel as a rule to differentiate it from the tapered thread). It has a constant diameter, a 55-degree angle, and rounded peaks and valleys.
BSPP male and female threads are always parallel, i.e., assembled with the same cylindrical shape. For these types of threads, it is recommended to use a sealant.
BSPT thread
BSPT thread is a tapered thread of the BSP standard (the T at the end stands for Taper as a rule to differentiate it from the parallel thread). The diameter of this pipe thread varies along the length of the thread. BSPT thread has the same 55-degree angle and rounded peaks and valleys.
BSPT male and female threads compress and wedge themselves together. As a result, these connections are more rigid, leakproof, and do not require sealant use.
NPT thread
NPT thread (National Pipe Taper) is a tapered thread of the American National Standard Pipe Thread standard. The diameter of this pipe thread varies along the length of the thread. NPT thread has a 60-degree angle and flattened peaks and valleys.
NPT thread connections also do not require sealant use, as the male and female tapered threads compress and wedge themselves together, providing leakproof connections.
The following figure shows the difference in shape between BSPP, BSPT, and NPT threads:
Are BSPP and BSPT threads compatible?
The golden rule is straightforward: a tapered male can fit a tapered or parallel female.
Therefore, a BSPT (tapered) male thread can fit both a BSPT (tapered) and a BSPP (parallel) female. However, a BSPP (parallel) male thread fits only a parallel (BSPP) female thread, as it is impossible to fit into a tapered female.
All this is reflected in the following figure:
Note that connecting a BSPT male to a BSPP female requires additional sealant to ensure a leakproof connection.
BSP and NPT threads are incompatible, as they have different numbers of threads per inch and thread angles. Attempting to join an NPT male to a BSPP or BSPT female will cause the joint to get stuck within a few turns. As a result, a spiral leak will occur because of the difference in thread shape.
BSP and NPT thread compatibility
| BSPP female | BSPT female | NPT female | |
| BSPP male | + | – | – |
| BSPT male | + | + | – |
| NPT male | – | – | + |
What connections require a thread adapter?
A thread adapter is required to connect the pipes having incompatible threads. Use the adapter with appropriate thread types to ensure rigid and leakproof connections.
For more information on compatible and incompatible pipe thread combinations, refer to the table above.
Which thread connections require sealant application?
A sealant is required for parallel, or straight, thread connections (e.g., BSPP).
Also, the sealant is required when connecting a tapered male thread to a parallel female thread (e.g., a BSPT male to a BSPP female).
The tapered thread connections (e.g., BSPT, NPT) do not require sealant use, as the male and female tapered threads compress and wedge themselves together, providing leakproof connections.
