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Rp vs. G British Standard Pipe Parallel

Rp vs G screw thread interchangeability

Similarities
The G-series and the Rp-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. Both have the matching size designations (nominal major diameter and threads-per-inch).

Differences
The differences between the G-series screw thread and the Rp-series screw thread include:
The G-series screw thread is made to ISO-228 while the Rp-series screw thread is made to ISO-7 or BS-21 or EN 10226-1.
The G-series screw thread may be internal screw thread or external screw thread; where the Rp-series screw thread is only internal screw thread.
The G-series screw thread is not intended for a seal being made on the screw thread where the Rp-series screw thread is specifically designed to seal when mated with an R-series tapered external 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: 

Chart: G vs Rp PD

Comparison of Minor Diameters
There is more to the screw threads than the effective pitch diameters. Because the Rp screw thread is designed to seal on the mating R external thread it has a much tighter tolerance and care must be taken when machining the Rp-series to assure that the minor diameters are within tolerance. Below is a chart displaying the G1” minor diameters compared with the Rp1” minor diameters:

Chart: G vs Rp MD

A Word on Gages
We recommend that you use gages which are defined in a specification. In this case:
- BS-21 Rp internal screw threads should be checked with BS-21 Rc Tapered Screw Thread Plug Gage.
- EN-10226 Rp-series internal screw threads should be checked with EN-10226 Rc Tapered Screw Thread Plug Gages: EN-10226 Gage-1* AND EN-10226 Gage-2*.
- ISO-7 Rp-series internal screw threads should be checked with ISO-7 Rc Tapered Screw Thread Plug Gages: ISO-7 Gage-1* AND ISO-7 Gage-2*.
- ISO-228 G-series internal screw threads should be checked with G-series Screw Thread Plug Gauges.
* EN-10226 gages are interchangeable with ISO 7 gages.

There are two unsanctioned screw thread plug gage options for testing the Rp screw thread:
1. Specially made Rp-series GO/NOGO parallel screw thread plug gages. These gages are not defined in any specification.
2. The same size designated G-series GO/NOGO screw thread plug gage. This gage cuts in half your Rp screw thread working tolerance, but it may be more readily available.
If you decide to use either of these unsanctioned GO/NOGO screw thread plug gages for testing Rp screw threaded parts; be ready to defend your engineering logic if, or when, you are challenged for your choice.

If you do decide to use screw thread plug gages which are not the gages specified in the associated screw thread specifications; be aware that screw thread plug gages do a poor job of testing the product screw thread minor diameter, and usually they ignore the product screw thread minor diameter. The product screw thread minor diameter must be tested via an alternate measurement methods (example: caliper or plain plug gages).

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 from an Authorized Reseller and make your own engineering evaluation of the facts.

Comments:
Original Posting: 1/24/2013
Last Revision: 12/12/2017
Error corrections in, or comments about, the above data can be sent to: gageguy@gagecrib.com

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