BS-21 System A vs. System B Taper Pipe Thread Gauges

Most simply stated, the difference between the System-A and the System-B BS-21 British Standard Taper Pipe Thread Gages is the way the gauges are read/used.

System A Screw Thread Gages
The System-A British Standard Taper Pipe Screw Thread Gauges have a single gauge plane which is used as the measurement reference point. The leading edge of the BS-21 tapered product screw thread must fall within plus or minus 1-1/4 turns from the notch or surface of the British Standard Taper Pipe Screw Thread Gage. If you notice that this method of reading the gauge is similar to the NPT series of gages which is read/used with plus or minus 1 turn from the notch or edge of the NPT gauge. Because of that, the British Standard System A is a favored by the USA gage makers.

For the System-A British Standard Taper Pipe Screw Plug Gage the gauge plane is a notch at the large end of the British Standard Taper Pipe Screw Plug Gage. The British Standard Taper Pipe Screw Plug Gauge is applied to the product screw thread which is gauged based on how the end of the product screw thread presents itself to the British Standard Taper Pipe Screw Plug Gage. To correctly use/read the British Standard Taper Pipe Screw Plug Gauge the end of the product screw thread must fall flush to the gage plane within +/-1.25 turns.

For the System-A British Standard Taper Pipe Screw Ring Gage the gauge plane is the surface of the large end of the British Standard Taper Pipe Screw Ring Gage. The British Standard Taper Pipe Screw Ring Gage is applied to the product screw thread which is gauged based on how the end of the product screw thread presents itself to the British Standard Taper Pipe Screw Ring Gage. To correctly use/read the British Standard Taper Pipe Screw Ring Gauge the end of the product screw thread must fall flush to the gage surface opposite of the gauge plane within +/-1.25 turns. Note that this is the surface at the small end of the British Standard Taper Pipe Screw Ring Gage instead of the gauge plane at the large side of the British Standard Taper Pipe Screw Ring Gage.

System-B Screw Thread Gages
The System-B British Standard Taper Pipe Screw Thread Gauges have the tolerance limits ground into the System-B British Standard Taper Pipe Screw Thread Gages as a step. One end of the System-B British Standard Taper Pipe Screw Thread Gauge is the maximum taper screw thread size and the step is the minimum taper screw thread size. The System-B British Standard Taper Pipe Screw Thread Gages have the steps marked with a plus (+) and a minus (-) to indicate the maximum and minimum product taper screw thread positions. The System-B British Standard Taper Pipe Screw Thread Gauge is applied to the product taper screw thread which is gauged based on how the end of the product taper screw thread presents itself to the System-B British Standard Taper Pipe Screw Thread Gage. The end of the product taper screw thread must fall between the end of the System-B British Standard Taper Pipe Screw Thread Gauge and the gage step. This system is considered the standard by the UK gauge makers, and is preferred elsewhere in the world. When we sell NPT taper screw thread gages to European customers they prefer the two-step design as described above.

System-B Plain Taper Gages
The BS-21 British Standard System-B also has defined System-B British Standard Plain Taper Pipe Gauges. The System-B British Standard Plain Taper Pipe Gages are used to check the taper of the screw thread at the crest of the product screw thread. One end of the System-B British Standard Plain Taper Pipe Gauge is the maximum product screw thread size and the gauge step is the minimum product screw thread size. The System-B British Standard Plain Taper Pipe Gages have the steps marked with a plus (+) and a minus(-) to indicate the maximum and minimum product screw thread positions. The System-B British Standard Plain Taper Pipe Gauge is applied to the product screw thread and the product is gauged based on how the end of the product screw thread presents itself to the System-B British Standard Plain Taper Pipe Gage. The end of the product screw thread must fall between the end of the System-B British Standard Plain Taper Pipe Gauge and the gage step. The System-B British Standard Plain Taper Pipe Gauges are to be used in addition to the System-B British Standard Taper Screw Thread Pipe Gages; not in place of them.

BS-21 vs. ISO-7
The product screw thread of the ISO-7 is technically equivalent to the product screw thread of BS-21. The ISO-7 screw thread gages are different than the BS-21 screw thread gauges because of differences in the gage design paradigms used by different standards bodies. Because of these two facts, two conclusions can be made. First; BS-21 screw thread gauges may be used to measure ISO-7 screw threaded product, and ISO-7 screw thread gages can be used to measure BS-21 screw threaded products. Second; in a gauge dispute all parties should harmonize the screw thread gages to the same standard. The use of screw thread gauges all made to the same standard may not resolve the dispute, but it should make the resolution easier.

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Original Post: 11/22/2011
Last Revised:
Comments or corrections may be sent to Wayne at:gageguy@gagecrib.com