The difference between BSP & NPT fittings is the threads per inch, shape and angle the threads are cut on. At some sizes threads per inch are very similar but there are other differences that make the threads incompatible.
Watch Huntley explain the differences that make the threads incompatible and the physical differences to decipher which is which.
![](https://s2.save4k.ru/pic/YORxZO9aZwI/maxresdefault.jpg)