In the field of piping connections, bspt vs npt are two standards that are often compared. These two threads, if confused, can lead to leaks in piping systems. So what are the core differences between bspt vs npt thread pipe? How do you choose between them? Next, the article will analyze their differences.
BSPT Vs NPT Thread, What Is Their Differences
BSPT Vs NPT Thread Definition
The full name of BSPT is British System Taper Pipe Thread. BSPT thread was first invented by British engineer Joseph Whitworth in 1841, and now it has become the ISO 7-1 international standard.
The full name of NPT is American Taper Pipe Thread, NPT thread was designed by American engineer William Sellers in 1864, mainly following ANSI B1.20.1 standard. These two standards are widely used in machinery manufacturing, petrochemical and other industries.
Core Differences
- Thread Angle
BSPT adopts 55 degree Whitworth thread angle
NPT uses 60 degree Sellers thread angle
This 5 degree difference directly affects the sealing performance and is the main reason why the two types of threads are not interchangeable.
- Thread Structure
BSPT threads have rounded tops and bottoms, a design derived from early British military standards. The NPT thread adopts a flat-topped profile, which is more convenient for sealing with Teflon tape.
Case in point: A chemical plant mistakenly screwed a BSPT connector into an NPT port, and although it could barely be screwed in, the mismatch in the tooth pattern resulted in high pressure gas leakage.
- Sealing Principle
BSPT relies on the deformation of the metal threads to achieve a seal.
NPT needs to be used in conjunction with a sealant material.
This explains why AFSL piping repair kits always include a sealant, whereas European fittings do not.
- Teeth Per Inch
Here is an example using a 1/2″ pipe diameter:
BSPT is 14 teeth per inch (pitch 1.814mm)
NPT is 14.5 teeth per inch (pitch 1.411mm)
Small differences in pitch can result in incomplete thread engagement.
BSPT Thread Vs NPT Dimensions Chart
BSPT Thread Dimensions Chart
Nominal Diameter | Thread Code | TPI | Pitch (mm) | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Taper Ratio | Effective Length (mm) | Pre-drill Size (mm) |
1/16″ | R1/16 | 28 | 0.907 | 7.723 | 7.142 | 6.561 | 1:16 | 6.5 | 6.4 |
1/8″ | R1/8 | 28 | 0.907 | 9.728 | 9.147 | 8.566 | 1:16 | 7.0 | 8.4 |
1/4″ | R1/4 | 19 | 1.337 | 13.157 | 12.301 | 11.445 | 1:16 | 9.5 | 11.2 |
3/8″ | R3/8 | 19 | 1.337 | 16.662 | 15.806 | 14.950 | 1:16 | 10.5 | 14.75 |
1/2″ | R1/2 | 14 | 1.814 | 20.955 | 19.793 | 18.631 | 1:16 | 13.5 | 18.25 |
3/4″ | R3/4 | 14 | 1.814 | 26.441 | 25.279 | 24.117 | 1:16 | 14.5 | 23.75 |
1″ | R1 | 11 | 2.309 | 33.249 | 31.770 | 30.291 | 1:16 | 17.8 | 30.0 |
1¼” | R1¼ | 11 | 2.309 | 41.910 | 40.431 | 38.952 | 1:16 | 18.0 | 38.5 |
1½” | R1½ | 11 | 2.309 | 47.803 | 46.324 | 44.845 | 1:16 | 18.5 | 44.5 |
2″ | R2 | 11 | 2.309 | 59.614 | 58.135 | 56.656 | 1:16 | 19.0 | 56.0 |
Note:
The formula for British Standard Pipe Thread (BSPT) is
Tooth Height H1 = 0.6403 x Pitch
Bottom Hole Size = Diameter + 0.05~0.15mm (adjusted according to machining allowance).
Tolerance range: Tolerance of ±0.08mm for center diameter. tolerance of ±1.0mm for effective length.
NPT Thread Dimensions Chart
Nominal Size | TPI | Pitch (mm) | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Hand Tight Engagement (mm) | Effective Thread Length (mm) |
1/16″ | 27 | 0.941 | 7.895 | 7.142 | 6.389 | 4.064 | 6.5 |
1/8″ | 27 | 0.941 | 10.242 | 9.489 | 8.736 | 4.572 | 7.0 |
1/4″ | 18 | 1.411 | 13.616 | 12.487 | 11.358 | 5.785 | 9.5 |
3/8″ | 18 | 1.411 | 17.055 | 15.926 | 14.797 | 6.096 | 10.5 |
1/2″ | 14 | 1.814 | 21.223 | 19.772 | 18.321 | 8.128 | 13.5 |
3/4″ | 14 | 1.814 | 26.568 | 25.117 | 23.666 | 8.611 | 14.5 |
1″ | 11.5 | 2.209 | 33.228 | 31.461 | 29.694 | 10.160 | 17.8 |
1-1/4″ | 11.5 | 2.209 | 41.985 | 40.218 | 38.451 | 10.668 | 18.0 |
1-1/2″ | 11.5 | 2.209 | 48.054 | 46.287 | 44.520 | 10.668 | 18.5 |
2″ | 11.5 | 2.209 | 60.092 | 58.325 | 56.558 | 11.074 | 19.0 |
Note:
NPT Thread formula:
Medium Diameter = Large Diameter – 0.8 x Pitch
Small Diameter = Large Diameter – 1.6 x Pitch
Dimensional tolerances are Dimensional tolerances are: Large diameter tolerance: ±0.13mm Medium diameter tolerance: ±0.08mm Effective length tolerance: ±1.0mm.
