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Class A vs. B British Standard Pipe Parallel

Class A vs. Class B

This page discusses the G-series screw thread as defined by ISO-228. Other older and obsolete references to this screw thread are: BS-2779; BSPP; BSP; BSPF; BSPM.

Internal G-Series Screw Thread
The internal G-series screw thread is not assigned a class-of-fit, and thus has no class-of-fit letter designated.

External G-Series Screw Thread
The class-of-fit designation only applies to the external G-series screw thread and by association the ring gages. The ISO-228 screw thread specification offers a choice of two pre-designed classes-of-fit for the male G-series screw thread: Class-A and Class-B.

What are the similarities?
In the ISO-228 G-series screw thread standard both classes-of-fit have the same basic size. They are identical. This means that the maximum material size is identical. Since the GO gauge checks the maximum material size, the GO gage is identical for both the Class-A and the Class-B. The GO gauge then can accurately be marked: A; B; A/B; or it can be left undesignated.

What are the dissimilarities?
In the ISO-228 G-series screw thread standard the minimum material condition is affected by the selection of the class-of-fit. The Class-A produces a tighter tolerance product, the Precision Class. The Class-B has twice the product tolerance of the Class-A and would be considered the commercial class.

Selection of Class-A may increase screw thread product production costs. It is common that the tighter the tolerance the higher the cost of manufacturing. Conversely, there may be a cost saving on the purchase of the gages. The Class-A NOGO gauge is usable to measure product that will pass both Class-A and Class-B requirements. For this reason the gage makers frequently stock only Class-A NOGO screw thread ring gauges.

Selection of Class-B may decrease screw thread product production costs. Again it is common that the wider the tolerance the lower the cost of manufacturing. Conversely, because the Class-B NOGO gage is only applicable to measure screw thread product designated as Class-B. For this reason gauge makers sell less of the Class-B NOGO screw thread ring gage and thus designate the Class-B screw thread ring gauges as a non-stocked special gage, with the expected higher price and longer made-to-order lead-time.

In Conclusion
In conclusion, here is a suggested G-series screw thread ring gauge purchasing strategy. Purchase the GO G-series screw thread ring gage and use it for either Class-A or Class-B regardless of how it is marked. If your requirement calls for Class-A; purchase the Class-A NOGO screw thread ring gauge. If your requirement calls for Class-B; and you are only making a few parts; purchase Class-A NOGO screw thread ring gage. If your requirement calls for Class-B; and you have larger production runs; purchase Class-B NOGO screw thread ring gauge. If you have trouble keeping track of this; buy both classes-of-fit screw thread ring gages as your screw thread product requirements dictate.

If you comment on this information contact Wayne at:gageguy@gagecrib.com
Last Revision: 2/23/2010

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