TECHNICAL ENGINEERING GUIDE

R, Rp, Rc & G Pipe Threads

The practical difference between BSPT, BSPP, ISO 7 and ISO 228 — explained clearly for engineers, buyers, manufacturers and instrumentation professionals.

The confusion in one sentence: R is tapered male, Rp is parallel female in the ISO 7 pressure-tight system, Rc is tapered female, while G is a parallel fastening thread under ISO 228.

Why does an Rp female thread remain parallel while working with a tapered R male? Is G 1/2 the same as Rp 1/2? Why can the basic dimensions look identical while the standards are different? And what does an Rp GO/NO-GO gauge actually prove? This guide answers those questions without relying on vague “BSP” terminology.

The goal is simple: after reading this, you should be able to identify the thread, understand its sealing principle, read the dimensions correctly and specify the right thread on an RFQ or drawing.

1. The First Question: Which Standard?

Pipe threads become confusing because the same nominal size appears in more than one system. A customer may write “1/2 BSP female,” but that wording does not tell you whether the intended thread is Rp 1/2, Rc 1/2 or G 1/2.

The first distinction is functional. ISO 7 covers pipe threads where the pressure-tight joint is made on the threads. ISO 228 covers parallel fastening pipe threads where the pressure-tight joint is not made on the thread itself.

This is why the phrase “parallel thread” is incomplete. Rp and G are both parallel, but they are governed by different standards and serve different functions. Likewise, a tapered female ISO 7 thread is Rc, not Rp.

Memory rule: R = male + tapered. Rp = female + parallel. Rc = female + tapered. G = parallel fastening thread.

2. R, Rp, Rc and G — The Definitions

R — external tapered: R is the ISO 7 designation for an external tapered pipe thread. In everyday industry it is commonly called BSPT male. Its diameter changes progressively along the threaded length.

Rp — internal parallel: Rp is an internal/female parallel thread in the ISO 7 pressure-tight system. It is specifically important because it mates with an R tapered male. Rp itself is not tapered.

Rc — internal tapered: Rc is the internal/female tapered member of the ISO 7 system. If the female thread itself is tapered, Rc is the designation to look for.

G — parallel fastening: G is the ISO 228 designation for a parallel thread, available internally or externally. Its sealing arrangement is outside the thread rather than being made by the thread itself.

A useful habit is to separate gender from geometry. That prevents the common mistake of calling every BSP female thread “BSPT female” or assuming that every parallel female thread is G.

3. Rp 1/2 vs G 1/2 — Same Basic Geometry, Different Specification

Rp 1/2 and G 1/2 can look almost identical because their basic 1/2 in thread geometry is the same: 14 TPI, approximately 1.814 mm pitch, and a 55° Whitworth thread form. Their basic diameters are also approximately the same.

So the correct engineering answer is not “Rp and G have completely different dimensions.” Their basic geometry can be the same. The difference is in the standard’s limits, gauging rules and intended function.

This also explains why simply measuring an old component with a caliper is not enough to identify it. A 1/2 in pipe thread has an actual basic major diameter of about 20.955 mm; the nominal 1/2 in label is not the measured outside diameter.

Same basic geometry ≠ same specification. Never substitute Rp 1/2 and G 1/2 simply because they screw together or measure similarly.

4. How Can a Parallel Rp Thread Seal With a Tapered R?

This is the key concept. Rp does not become tapered. It remains a parallel female thread. The pressure-tight behaviour comes from the specified mating system: tapered R male with the controlled Rp female thread.

R male → Rp female

As the tapered R male enters the Rp female, the relationship between the mating thread surfaces changes progressively with engagement. The design of the standardized pair controls how the threads come into their working relationship.

Correct assembly and appropriate thread-sealing practice are also important in real applications.

Compare that with G. A G male and G female are parallel. Under ISO 228, the thread is intended for fastening; the pressure-tight seal is made by a separate sealing surface, such as a gasket, washer or O-ring arrangement appropriate to the component.

This is why saying “Rp is BSPP” without further explanation can create confusion. Rp is parallel, but its standard and function are not the same as G.

Correct statement: “Rp is a parallel female thread designed for the ISO 7 pressure-tight threaded system with an R tapered male thread.”

5. Dimensions, Tolerances and Gauge Values

A thread standard contains more than one set of numbers. Engineers often mix up the basic thread dimensions, the workpiece limits and the gauge dimensions. They are not automatically the same thing.

For 1/2 in BSP-family geometry, the basic values are approximately 20.955 mm major diameter, 19.793 mm pitch diameter, 18.631 mm minor diameter and 1.814 mm pitch. These numbers are useful for understanding the thread, but they should not be treated as the final GO/NO-GO gauge dimensions.

A GO/NO-GO gauge is a precision inspection instrument. Its specified values are derived from the applicable gauging system and its purpose. Therefore, if a calibration certificate contains values such as 20.823 mm or 19.661 mm, those values should be checked as gauge-specific values, not automatically compared with the basic workpiece major or pitch diameter.

Remember: basic thread dimension → workpiece limits → gauge design → gauge tolerance → calibration result. Do not skip the middle steps.

6. Rp 1/2 GO / NO-GO Plug Gauge

An Rp 1/2 GO/NO-GO plug gauge is used to inspect an internal Rp 1/2 thread. Because the component is female, the inspection gauge has an external threaded form that enters the workpiece.

The GO side checks the required functional acceptance condition. The NO-GO side checks the opposite limit. The gauge is therefore not simply a convenient “thread-shaped measuring tool”; it is part of a defined limit-gauging system.

This distinction matters when a customer asks for the source of the specified values on a calibration certificate. The laboratory may show actual readings within the certificate’s printed tolerance, but the customer can still ask why those printed tolerances and nominal values were selected.

For the ISO 7 system, ISO 7-1 covers the pipe-thread dimensions, tolerances and designations, while ISO 7-2 covers verification by limit gauges. When a certificate uses an older edition, the required standard edition should be confirmed with the customer or calibration laboratory before declaring it compliant with a current specification.

7. Quick Answers Engineers Need

Is Rp tapered?

No. Rp is an internal parallel thread.

Is R tapered?

Yes. R is an external tapered ISO 7 thread.

Is Rc tapered?

Yes. Rc is an internal tapered ISO 7 thread.

Is G tapered?

No. G is a parallel ISO 228 fastening thread.

Is G 1/2 the same as Rp 1/2?

They can share the same basic geometry, but they are governed by different standards and have different functional and gauging requirements.

Can R 1/2 mate with Rp 1/2?

Yes. R male and Rp female form a standard ISO 7 pressure-tight threaded combination when correctly manufactured and assembled.

Does Rp itself become tapered during assembly?

No. Rp remains parallel; the sealing behaviour comes from the specified R + Rp mating system.

What does BSPP normally mean?

In common industry usage, BSPP normally refers to the parallel G thread family.

What does BSPT normally mean?

In common industry usage, BSPT normally refers to the tapered BSP family. ISO terminology distinguishes R male and Rc female; Rp is the parallel female ISO 7 member.

Is BSPT the same as NPT?

No. BSP/ISO pipe threads use a 55° Whitworth form, while NPT uses a 60° form.

Why is 1/2 BSP about 21 mm?

Because 1/2 in is a nominal pipe-thread size. The basic major diameter is about 20.955 mm.

Can a G gauge be used to inspect Rp?

Do not assume so. Use the gauge system required by the governing standard and drawing.

What is the most important information missing from “1/2 BSP female”?

Confirm whether the requirement is Rp 1/2, Rc 1/2 or G 1/2, and state the applicable standard.

The complete mental model: R = tapered male. Rp = parallel female in ISO 7. Rc = tapered female in ISO 7. G = parallel fastening thread in ISO 228. Once standard, geometry and sealing function are separated, the confusion largely disappears.

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Use the exact standard edition specified by the customer, drawing or inspection plan. This guide is an engineering explanation and should not replace the applicable controlled standard for production inspection.