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. In all situations the G-series can be used when the Rp-series is specified, but the reverse is not true.
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:
Notice how the G-series will always be within the specified parameters for the Rp-series, but the Rp-series may be smaller than the G-series. Based on this the G-series is always useable when a Rp-series is requested, but an Rp-series may not work when a G-series is requested. The reason for this is that both the G-series screw thread and the Rp-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 positive and none applied to the minus of the Basic Effective Pitch Diameter. The Rp-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 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.
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.
Original Posting: 1/24/2013
Last Revision: 9/29/2017
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