G 是 55 度非螺纹密封管螺纹，属惠氏螺纹家族．标记为 G 代表圆柱螺纹．母G是Gas的缩写（一说是源于德语Gewinde，“螺纹”的意思，博主个人比较倾向于前者），因为起初是在煤气管道上用的。而我们之所以称G为管螺纹，是因为G和管（Guan）的拼音首字母相同的缘故，这实在是一个巧合。G螺纹分内、外两种螺纹，用于非密封联接。执行标准是BS EN ISO 228-1，旧标准是BS 2779，现已作废。 

British Pipe Thread Gauges

·        G Series British Standard Pipe – Parallel ( Straight ) BSP or BSPF

·        Also referred to as British Gas, British Pipe Parallel or Parallel Fastening thread.

·        Various Symbols: (G) BSPT, BSP, BSPP, BSSPI, BSPF, BSPG, PS, R, G, BSPTr, PT, KR, Rc.

·        There are two classes of external threads: “A” (close fit) and “B” (free fit).

·        Class “B” is used unless Class “A” is specified.

·        Call for Class “A” ring and set plug gage pricing  System A and the threads are modified teeth, not full form such as British Whitworth. .

BSPP means it is a straight thread.
Modified – Plugs Major diameter
Rings – Minor Diameter
Also, provided medium class from BS2779.

G is the full form- where the Go Gages has radius.
Rings – Class A or B
Plug No Class
ISO 228

BSPT is a tapered thread that is modified again
System A or B
RC/R is a tapered thread Full Form
System A or B
BS 21

苏州斯文克精密测量设备有限公司offers a complete line of British parallel pipe threads per ISO 228 and BS ‐2779. In March 2003 the British Standards Institute withdrew BS‐2779 and replaced it with ISO‐228. ISO 228 ‐ 1 covers pipe threads where pressure‐tight joints are not made on the threads. ISO 228 ‐ 2 covers the limit gages for pipe threads where pressure‐tight joints are not made on the threads. ISO 1179, pipe connection, threaded to ISO 228‐1 and gauged with ISO 228 ‐ 2 gauges for plain end steel and other metal tubes in industrial applications. BSPP Thread gages have a 55 degree thread profile. The specification covers several Go and several No Go gauges designed to verify different elements of the thread. The threaded Go gages ensure that the profile of the machined piece does not exceed the maximum of material provided for by the tolerances applied to the dimensions of the profile defined by ISO 228‐1. The threaded No Go gauges for the threads of the pieces fix the minimum material limit on the flanks of the thread. GSG offers both the full form style as called out in the ISO specification as well as a modified version manufactured in the U.S.A. Full form thread ring gages are designed in a solid style and may be calibrated with a wear check plug gage. Ring gages are available in Class A and B. Class A Go ring gages are also valid for class B threads. No Go class B rings are quoted as special.

ISO Standard: Basic Dimensions: ISO 228 (1) – 2000 & Gauging Practice: ISO 228 (2) – 1987.
British Standard – BS 2779 – 1986.
Indian Standard – Basic Dimensions: IS 2643 – 2005 &  Gauging Practice: IS 10216-1988.
Japanese Standard: JIS B 0202-1999.