Extended data:
English thread is marked with the size of English thread, which is divided into cylindrical and conical according to its shape; According to the angle of tooth type, there are 55 and 60. The marks 1/4, 1/2 and 1/8 in the thread refer to the diameter of the thread size in inches. One inch equals eight points, 1/4 inches equals two points, and so on.
China's pipe threads basically follow international standards and adopt English system. Because British standards compete with American standards, foreign standards are divided into 55 degrees and 60 degrees. In the past, China only divided pipe threads into G and Z, and this usage is still in use in some factories. The hole marks of some pipe fittings, such as G 1/2 for 55-degree cylindrical pipe thread, Z 1/2 or ZG 1/2 for 55-degree conical pipe thread, and Z 1/2.
At present, the classification method is that 55-degree unsealed pipe thread is G, and 55-degree sealed pipe thread has two types: Rp cylindrical internal thread with conical external thread R 1 and Rc conical internal thread with conical external thread R2, and 60-degree conical pipe thread, namely NPT pipe thread, is marked as NPT 1/2.
References:
Baidu encyclopedia-inch thread