In the field of pipes, pipe materials, and fittings, understanding the different types of thread standards is crucial to ensuring proper connections and avoiding leaks. The two most commonly used thread standards worldwide are BSP (British Standard Pipe) and NPT (National Pipe Thread). Although their purposes are similar, they have different characteristics and applications. ZIZI explains the subtle differences between BSP and NPT threads to help clarify their specific uses in different situations.
BSP Threads
BSP threads are divided into three types: BSPP (British Standard Pipe Parallel), BSPT (British Standard Pipe Taper), and BSPF (British Standard Pipe Fine).
BSPT (Tapered): BSPT threads have a tapered profile, creating a seal through the wedging action between the external and internal threads. Sealing is typically enhanced using thread sealants such as PTFE tape or pipe sealant. BSPT threads are commonly used in applications requiring a tight seal, including hydraulic and pneumatic systems.
BSPF (parallel): These have parallel threads that rely on an adhesive seal (using a washer or O-ring) placed between the male and female connector ends to prevent leakage. BSPF threads are typically used in applications where pressure-resistant sealing is less critical or where external sealing mechanisms are preferred.
BSPP (British Standard Parallel Thread) is a 55° parallel thread for sealed connections. In the VPC series of tube fittings, BSPP threads are equipped with O-rings by default.
BSP Threads Chart
| Pipe size | Thread number in 25.4mm | Thread pitch (P) | Thread height (h) | Diameter at gauge plane | Nominal gauge length (external thread) | ||||
| Major gauge diameter (d=D) | Pitch (d₂=D₂) | Minor (d₁=D₁) | |||||||
| (inch) | (mm) | (no.) | (mm) | (mm) | (mm) | (mm) | (mm) | (inch) | (mm) |
| 3/8″ | 17.2 | 19 | 1.337 | 0.856 | 16.662 | 15.806 | 14.95 | 1/4 | 6.4 |
| 1/2″ | 21.3 | 14 | 1.814 | 1.162 | 20.955 | 19.793 | 18.631 | 0.3214 | 8.2 |
| 3/4″ | 26.9 | 14 | 1.814 | 1.162 | 26.441 | 25.279 | 24.117 | 3/8 | 9.5 |
| 1″ | 33.7 | 11 | 2.309 | 1.479 | 33.249 | 31.770 | 30.291 | 0.4091 | 10.4 |
| 1- 1/4″ | 42.4 | 11 | 2.309 | 1.479 | 41.910 | 40.431 | 38.952 | 1/2 | 12.7 |
| 1- 1/2″ | 48.3 | 11 | 2.309 | 1.479 | 47.803 | 46.324 | 44.845 | 1/2 | 12.7 |
| 2″ | 60.3 | 11 | 2.309 | 1.479 | 59.614 | 58.125 | 56.656 | 5/8 | 15.9 |
| 2- 1/2″ | 76.1 | 11 | 2.309 | 1.479 | 75.184 | 73.705 | 72.226 | 11/16 | 17.5 |
| 3″ | 88.9 | 11 | 2.309 | 1.479 | 87.884 | 86.405 | 84.926 | 13/16 | 20.6 |
| 4″ | 114.3 | 11 | 2.309 | 1.479 | 113.030 | 111.551 | 110.072 | 1 | 25.4 |
| 5″ | 139.7 | 11 | 2.309 | 1.479 | 138.430 | 136.951 | 135.472 | 1-1/8 | 28.6 |
| 6″ | 165.1 | 11 | 2.309 | 1.479 | 163.830 | 162.351 | 160.872 | 1-1/8 | 28.6 |
NPT Threads
The National Pipe Thread standard, abbreviated as NPT, is a U.S. national standard for pipe threads, with a thread angle of 60°. Based on thread shape, it is divided into sealing straight pipe threads and sealing tapered pipe threads. Among these, cylindrical internal threads and conical external threads form a “cylinder/cone” fit, while conical internal threads and conical external threads form a “cone/cone” fit. Different shapes of male and female threaded ends also vary, such as NPT (National Pipe Taper), NPS (National Pipe Straight), NPTF (National Pipe Taper for Fuel and Oil), or NPSC (National Pipe Straight Coupling), etc. NPT threads are characterized by excellent sealing performance and are commonly used for conveying liquids, gases, steam, and hydraulic fluids.
NPT Threads Chart
| Pipe size | Thread number | Thread pitch | Thread height | Hand-tight engagement | Effective thread | |||||
| D | No. | P | h | Length | Threads | Diameter | Length | Threads | Diameter | |
| (inch) | (mm) | (no./inch) | (inch) | inch | (L. inch) | (no.) | (E₁.inch) | (L2, inch) | (no.) | (E₃ inch) |
| 3/8″ | 17.1 | 18 | 0.05556 | 0.04444 | 0.240 | 4.32 | 0.62701 | 0.4078 | 7.34 | 0.63750 |
| 1/2″ | 21.3 | 14 | 0.07143 | 0.05714 | 0.320 | 4.48 | 0.77843 | 0.5337 | 7.47 | 0.79178 |
| 3/4″ | 26.7 | 14 | 0.07143 | 0.05714 | 0.339 | 4.75 | 0.98887 | 0.5457 | 7.64 | 1.00178 |
| 1″ | 33.4 | 11.5 | 0.08696 | 0.06957 | 0.400 | 4.60 | 1.23863 | 0.6828 | 7.85 | 1.25631 |
| 1- 1/4″ | 42.2 | 11.5 | 0.08696 | 0.06957 | 0.420 | 4.83 | 1.58338 | 0.7068 | 8.13 | 1.60131 |
| 1- 1/2″ | 48.3 | 11.5 | 0.08696 | 0.06957 | 0.402 | 4.83 | 1.82234 | 0.7235 | 8.32 | 1.84131 |
| 2″ | 60.3 | 11.5 | 0.08696 | 0.06957 | 0.436 | 5.01 | 2.29627 | 0.7565 | 8.7 | 2.31630 |
| 2- 1/2″ | 73.0 | 8 | 0.12500 | 0.10000 | 0.682 | 5.46 | 2.76215 | 1.1375 | 9.1 | 2.79063 |
| 3″ | 88.9 | 8 | 0.12500 | 0.10000 | 0.766 | 6.13 | 3.3885 | 1.2000 | 9.6 | 3.41563 |
| 4″ | 114.3 | 8 | 0.12500 | 0.10000 | 0.844 | 6.75 | 4.38712 | 1.3000 | 10.4 | 4.41563 |
| 5″ | 141.3 | 8 | 0.12500 | 0.10000 | 0.937 | 0.94 | 5.44929 | 1.4063 | 11.25 | 5.47863 |
| 6″ | 168.3 | 8 | 0.12500 | 0.10000 | 0.958 | 7.66 | 6.50597 | 1.5125 | 12.1 | 6.54063 |
BSP vs NPT Threads
Threads Angle
This is one of the most obvious differences between the two. BSP is a 55° British standard pipe thread with a rounded structure at the top and bottom of the thread. NPT is a 60° American standard tapered pipe thread with a tapered profile and a flat structure at the top and bottom of the thread. This difference makes the two threads incompatible for direct interchangeability, and the corresponding thread cutting tools and gauges are also completely different. These differences in shape and angle result in their unique sealing performance and applications. They also affect the compatibility and interchangeability of these threads, so careful consideration is required during design and assembly.
Sealing Method
Both NPT and BSP are tapered threads, meaning they achieve sealing by forming metal-to-metal contact and compression through the thread’s taper when tightened. However, subtle differences in thread design affect their sealing characteristics and the need for additional sealants.
Although NPT design is intended to achieve sealing through thread contact, due to thread manufacturing tolerances and the helical path between the thread peaks and valleys, it is strongly recommended to use thread sealant (such as PTFE tape, commonly known as Teflon tape, or pipe thread sealant) to fill these microscopic gaps, ensuring a completely leak-free connection. This is particularly important in applications involving gases or high-pressure fluids.
BSP threaded connections also typically require thread sealant to ensure reliable leak prevention. While its curved thread profile may theoretically help distribute stress more effectively, sealant remains indispensable in practical applications, especially in high-pressure or sensitive fluid systems.
Measurement Units
NPT thread sizes are specified in inches, such as 1/8“, 1/4”, 1/2“, 1”, etc. These numbers represent the nominal pipe size rather than the actual thread diameter. For example, the outer diameter of a 1/2″ NPT thread is approximately 0.84 inches (approximately 21.3 millimeters).
BSPT threads also use inches as nominal sizes, such as 1/8“, 1/4”, 1/2“, 1”, etc. Similarly, these numbers are nominal pipe sizes and do not directly correspond to measured diameters. For example, the outer diameter of a 1/2″ BSPT thread is approximately 0.825 inches (approximately 20.96 millimeters).
Advantages and Disadvantages of NPT Threads
Advantages:
The tapered design provides strong sealing performance, making it suitable for high-pressure applications.
The NPT thread design is specifically engineered to achieve sealing through thread contact, eliminating the need for additional O-rings or gaskets.
It prevents sealant contamination of the medium, making it suitable for sensitive applications such as fuel and lubricants.
Strict dimensional parameters ensure good interchangeability between products from different manufacturers, making them widely compatible in North America and regions adhering to related standards.
Disadvantages:
High processing precision requirements necessitate specialized tools, resulting in higher processing costs.
Although designed for sealing, NPT threads still have spiral leakage paths between the thread peaks and valleys, so thread sealant is almost always required to ensure complete leak prevention.
Each disassembly and reassembly may cause some wear or deformation to the threads, reducing their sealing performance. In non-North American regions, its versatility is inferior to BSP threads, potentially increasing procurement and compatibility challenges.
Advantages and Disadvantages of BSP Threads
Advantages:
BSP is easy to install, relying on sealing elements for sealing, with lower requirements for installation skills.
BSP uses a 55-degree Whitworth thread angle, with rounded peaks and valleys, which helps reduce stress concentration.
BSP combines tapered sealing characteristics, providing stable sealing performance in medium to low-pressure scenarios; it has strong versatility in regions such as Europe and Asia, with convenient procurement and compatibility.
Disadvantages:
BSP has no inherent sealing capability and relies on additional sealing elements, increasing the number of parts and potential leakage points.
Like NPT, frequent disassembly and reassembly may wear the threads, affecting their sealing performance.
BSP has less stable sealing performance than NPT in high-pressure applications. Different types cannot be interchanged arbitrarily and must be strictly matched, increasing selection complexity.
Application Scenarios
Among BSP threads, BSPP is mostly used in low-pressure situations with low sealing requirements, such as household water supply pipes and ordinary pneumatic pipes; BSPT is more common in medium and low-pressure hydraulic systems, industrial cooling water pipes and other scenarios; BSPF is suitable for pipe connections of high-pressure hydraulic equipment and precision instruments due to its precision.
NPT threads are more used in high-pressure pipeline systems, hydraulic and pneumatic pipes in industrial equipment, gas pipes and other occasions with high sealing performance requirements, such as automotive brake systems, industrial compressor pipes, etc. NPTF threads are widely used in fuel pipes, lubricating oil systems, hydraulic pipes of aerospace equipment and other fields with extremely high requirements for sealing reliability and cleanliness.
Conclusion
In summary, the differences between BSP and NPT threads in terms of angle, thread profile, and sealing method determine their respective application ranges in different industrial sectors and regions. In practical use, the appropriate thread type must be selected based on specific requirements to ensure the sealing and reliability of pipe connections.
The most important point is: under no circumstances should NPT threads be mixed with BSPT threads. This will result in connection failure, leaks, or even safety hazards. When selecting pipe threads, it is essential to decide based on the standards in your region, the thread type of existing equipment, and specific application requirements (such as medium, pressure, temperature, and whether frequent disassembly is required).
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