R, Rc, G or NPT: identify the connection before ordering
A nominal size is only the start. Establish the thread on each side and where the joint is intended to seal.
Imagine a maintenance buyer requesting a replacement valve with a “1/2-inch connection.” One quotation says Rc 1/2, another says G 1/2, while the existing hose end is marked 1/2 NPT. The valve bodies look similar and a loose fitting appears to start in the port. That leaves the essential question unanswered: which two ends belong together, and what makes their joint seal? Before comparing prices, recover the existing connection specification and the proposed replacement’s complete port designation.
Read the port option behind the model number
A photograph of the equipment helps locate the connection, but its series name may not identify the port. Look for the complete model code, nameplate, drawing and ordering record. As a concrete manufacturer example, SMC’s CG5-S catalog distinguishes Rc, NPT and G port options under stated size and configuration conditions. The catalog demonstrates why the ordering suffix matters; it does not make those options interchangeable. [3]
Keep the equipment port and the removable fitting separate in your description. A valve may have an internal thread, the hose end an external thread, and an adapter between them. Mark each interface on a photograph. If “BSP” appears alone on an old order, ask which designation and connection drawing it means rather than expanding it into a purchase specification yourself. This keeps a familiar trade term from concealing an unknown interface.
The letters describe more than taper
In the ISO 7 context, R denotes an external tapered thread, Rc an internal tapered thread and Rp an internal parallel thread. Rp therefore belongs to the thread-sealing family despite being parallel. G is the parallel pipe-thread designation associated with ISO 228. A parallel female thread cannot be identified as Rp or G simply by looking at its straight outline. [2]
For a specified R/Rc or R/Rp connection, sealing is intended at the threads under the applicable connection requirements. A G-threaded connection needs its specified sealing arrangement outside the thread engagement, such as a suitable face seal, washer or seat. The sealing surface and its mating component must be identified along with the thread. Swagelok explicitly separates thread geometry from the complete end connection, a useful distinction when a parts list records only “G 1/2.” [1]
- 1Tapered pair: sealing at the threads
- 2Parallel pair: separate face seal
- 3Male and female ends are one connection
Schematic examples, not a compatibility chart. The left pair illustrates R/Rc; the right pair illustrates a G connection with a specified face seal. Rp is parallel but belongs to ISO 7 thread-sealing connections. Shape alone does not identify the standard or every sealing design.
NPT is a different thread, even when the size looks familiar
NPT is not another spelling for R. Its 60° thread form differs from the 55° form used in the ISO pipe-thread families discussed here. Diameter and pitch can look close, and some nominal sizes are particularly hard to distinguish. Swagelok calls out 1/2- and 3/4-inch ISO 7 and NPT threads as examples that may require optical comparison for positive identification. [1]
In the hypothetical valve order, being able to turn the hose end into a candidate port by hand would not settle that distinction. It also says nothing about the intended seal, useful engagement or the condition of the port. Keep the original parts identifiable while the specification is recovered. Forcing a trial assembly can damage the evidence and the equipment before a replacement decision has been made.
Measure to narrow the candidates, then verify the specification
Record whether each end is male or female, whether diameter changes along the thread, where the diameter was measured and which pitch gauge matches. Photograph the scale and measuring position. For pipe threads, a nominal designation such as 1/2 is not the outside diameter to expect from a caliper, nor does measuring a nearby diameter establish that nominal size. Measurements support identification when read against the correct reference.
Ask the supplier how the proposed thread will be verified against the agreed specification. A pitch gauge helps distinguish candidates; conformity requires the applicable thread gauges and other specified checks. The public scope of ISO 7-2 describes limit-gauge verification while explaining that gauges alone do not cover all thread quality and dimensions. Its full gauge procedure is outside the public material read here. [2]
| Available information | Still unresolved | Useful next evidence |
|---|---|---|
| “1/2-inch connection” | Still unresolvedThread family, male/female and seal are unknown. | Useful next evidenceFull model and port drawing for both ends. |
| Parallel female thread | Still unresolvedCould be Rp, G or another system. | Useful next evidenceSpecified designation, gauge evidence and sealing detail. |
| Tapered thread; pitch seems to match | Still unresolvedR and NPT are not distinguished by taper alone. | Useful next evidenceCorrect thread-form reference and appropriate verification. |
| G designation is known | Still unresolvedThe complete sealing interface is not specified. | Useful next evidenceSeal/seat drawing, matching face and included seal part. |
Specify the seal and operating conditions with the adapter
An adapter proposal should name both ends, including male/female, standard and nominal size, and show the sealing arrangement at each interface. On a face-sealed G connection, ask which seal is supplied and which mating face it requires. For a thread-sealing connection, obtain the selected product’s assembly and sealing instructions. Tapered versus parallel is an identification clue, not a complete installation instruction; specialist designs can use a separate seal even with a tapered thread. [1]
Provide the fluid, operating pressure and temperature, materials and any cleaning or environmental requirements. These conditions affect the assessment of the fitting, seal and equipment together. An adapter also changes the physical installation: its added length may obstruct a guard or move a hose into a tight bend. Compare the proposed assembly drawing with the available space before accepting a solution based only on thread labels.
Turn the answer into an order that can be checked
Return to the quotations. Once the existing hose end and valve port are established, ask each supplier to quote the same defined arrangement or to show its proposed change explicitly. An Rc valve with an approved transition assembly and a G valve with a specified face-seal assembly are different offers. Compare included fittings, seals, drawings and verification records with price and lead time.
Put the accepted model, port options, adapter and seal parts, drawing revision and agreed inspection or functional checks into the order. Treat missing identification or unresolved use conditions as open points, with an owner and a required answer before release. On receipt, match the delivered markings and documents to that order; any agreed leak or functional test remains a separate check under the applicable equipment procedure. The useful result is a traceable connection choice that the next replacement buyer can recover.
Inquiry text to adapt
Open supplier questions
Please review the connection for [equipment / complete model / drawing revision]. The existing port is [designation, standard, nominal size, female/male or unknown] and the mating fitting is [details or unknown]. Attached are [markings, end views, sealing surfaces and measurement locations]. The service is [fluid, pressure, temperature and materials], with delivery to [destination]. Please identify the proposed part and both adapter ends, their sealing arrangements, included seals and required mating surfaces. Explain the evidence for thread identification and the inspection records available. Please show any change in installation envelope or assembly requirements and identify unresolved information. Any proposed specification change should be submitted for approval before the order is released.
Sources and scope
- Swagelok — Thread and End Connection Identification Guide, MS-13-77 rev H
Manufacturer identification guide, selected terminology, identification and seal examples; not a dimensional acceptance specification or universal compatibility table.
- ISO 7-2:2000 — public OBP scope and contents
Official ISO public contents and scope. R/Rc/Rp headings and the bounds of gauge verification were read; the full standard and gauge dimensions were not.
- SMC — Stainless Steel Cylinder CJ5-S / CG5-S digital catalog
Specific CG5-S ordering options and dimensional drawings; conditions vary by cylinder size and configuration.
- ISO 7-1:1994 — official public catalog
Official ISO 7-1 catalog title and abstract only; no full normative thread or mating rules read.