1. First, look at the thread angle
The most accurate method is to measure with a thread gauge. 60° is for the American standard, and 55° is for the British standard.
If no tools are available, for pipe threads, you can observe the sealing method: American standard NPT threads usually seal by the taper of the thread itself; British standard BSP threads (such as BSPT, BSPP) have a thread angle of 55°, and the parallel BSPP threads need to rely on the sealing ring for sealing.
2. Key check parameters (dimensions and TPI)
For fasteners like bolts and nuts with threaded connections, after measuring the major diameter (outer diameter of the external thread / inner diameter of the internal thread), the most crucial thing is to count the number of threads per inch (TPI), and then check against the standard manual.
For pipe threads, in addition to measuring the dimensions, you also need to determine whether it is a tapered thread (such as NPT, BSPT) or a straight thread (such as BSPP), as well as the sealing method.
3. Directly check the standard code
If there is a standard marking on the part (such as UNC, UNF, BSW, BSF, NPT, BSPP, etc.), this is the most reliable basis.
Trace back to the source
The British standard thread originated from Joseph Whitworth’s 55-degree Whitworth thread in 1841;
The American standard thread was proposed by William Sellers in 1864 as a 60-degree thread.
In fact, during World War II, the British and American standards caused significant difficulties for the logistics exchange of equipment between the Allied forces. Therefore, in 1948, the United States, the United Kingdom, and Canada jointly developed a unified thread standard (UNC/UNF).
Interestingly, starting from 1961, relevant international organizations adopted the British standard thread specification, ISO R 228. So now, the British thread is a universally used hard standard worldwide.
Post time: Feb-05-2026
