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Rs vs. G British Standard Pipe Parallel

Rs vs G screw thread interchangeability

NOTE
The Rs-series per BS 21 is an obsolete thread series and should not be used in new designs; however; you may find a requirement for Rs-series screw threads on an older product/part design drawing.

Similarities
The G-series and the Rs-series British Standard Screw Threads have much in common:
a. Both are British Standard Pipe Parallel.
b. Both have the Whitworth 55° included thread angle form with Full Form rounded root and crest of thread.
c. In all situations the G-series can be used when the Rs-series is specified, but the reverse is not true.

Differences
The differences between the G-series screw thread and the Rs-series screw thread include:
The G-series screw thread is made to ISO-228 while the Rs-series screw thread is made to BS-21 Appendix C.
The G-series screw thread may be internal screw thread or external screw thread; where the Rs-series screw thread is only external screw thread.
The G-series screw thread is not intended for a seal being made on the screw thread where the Rs-series screw thread is specifically designed to seal when mated with an Rc-series tapered internal screw thread.

Comparison of Pitch Diameters
I think a basic understanding of the two screw threads is embodied in a comparison of their effective pitch diameters. Below is a chart displaying the G1” effective pitch diameters compared with the Rp1” effective pitch diameters: 

Rs vs G Pitch Diameters

Conclusion
Notice how the G-series will always be within the specified parameters for the Rs-series, but the Rs-series may be smaller than the G-series. Based on this the G-series is always useable when a Rs-series is requested, but an Rs-series may not work when a G-series is requested.The reason for this is that both the G-series screw thread and the Rs-series screw thread have the same Basic Effective Pitch Diameter. The Internal G-series screw thread then has an amount of tolerance applied to the minus and none applied to the positive of the Basic Effective Pitch Diameter. The Rs-series has the same amount of tolerance added to the positive side of the Basic Effective Pitch Diameter AND an equal amount applied as a minus from the Basic Effective Pitch Diameter.

A word on Gages
We reccommend that you use gages which are defined in a specification. In this case:
- BS-21 Rs external screw threads should be checked with BS-21 R-series Tapered Screw Thread Ring Gage.
- ISO-228 G-series external screw threads should be checked with G-series Screw Thread Ring Gauges.

Disclaimer
This information given is my opinion from my study of the standards. I have no authority and my opinion thus has no value. If you intend on acting on this information; I suggest that you purchase the official engineering documents: BS21 and ISO228 from an Authorized Reseller and make your own engineering evaluation of the facts.

Comments:
Original Posting: 1/24/2013
Last Revision: 1/27/2015
Error corrections in, or comments about, the above data can be sent to: gageguy@gagecrib.com

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