We often get confused about RC threads and PT threads: Are they the same thing? If not, where is the difference?
In fact, the answer to this question is not simply "yes" or "no"; it involves the evolution of thread standards and regional
naming conventions. To put it simply, in most cases, RC threads and PT threads refer to the same type of thread,
but there are historical origins in terms of standard attribution and naming.
I. Tracing the "Background" of RC and PT Threads
The PT thread is the abbreviation of "Pipe Thread", which originated from the industrial standard of Japan (JIS).
In the previous version of the JIS B 0203 standard, PT was used to refer to the sealing conical pipe thread with
a 55° tooth angle. This type of thread was widely used in early industrial equipment in Asian regions,
especially in Japan and South Korea. Therefore, many experienced masters habitually referred to this kind of
conical pipe thread as PT tooth.
The RC thread originated from the standards of the International Organization for Standardization (ISO).
The ISO 7-1 standard defines the pipe threads used for achieving sealing on the threads, where Rc represents a 55°
conical internal thread (BSPT - British Standard Pipe Taper). With the advancement of globalization, national standards
are gradually aligning with the ISO standards. The new JIS standards of Japan also adopted the same labeling method
as ISO, using R to represent the conical external thread and Rc to represent the conical internal thread. Therefore,
from the perspective of the modern standard system, RC is a more standardized and international term,
while PT can be regarded as the "former name" of the RC thread in a specific historical period and region.
II. Comparison of Core Technical Parameters: Analyzing from Data
Since they are essentially the same type of thread, their key technical parameters must be the same.
We will use several core data points to make it clearer for you.
1.Thread Angle
This is an important indicator for differentiating different thread systems. Whether it is RC thread or PT thread,
their thread angles are both 55°. This is fundamentally different from the NPT thread of the American standard,
whose thread angle is 60°. This 5° difference means that they must never be used interchangeably.
2. Taper
Both RC and PT threads belong to conical pipe threads. Their function is to achieve a sealing effect through the
conical surface of the threads by generating interference. The taper of both is 1:16, which means that for every 16
millimeters advancement along the axis of the thread, the diameter of the thread will change by 1 millimeter.
The same taper is the basis for their interchangeability.
3. Sealing Principle
Unlike G-thread (cylindrical pipe thread) which relies on gaskets or O-rings for sealing, the sealing of RC and PT
threads mainly depends on the conical metal surfaces of the internal and external threads. By tightening,
the sides of the thread teeth are made to closely adhere, generating sealing pressure. In practical applications,
to achieve a more reliable sealing effect, polytetrafluoroethylene (PTFE) tape or specialized thread sealant is usually
used to fill the possible tiny gaps between the metal contact surfaces.
III. Practical Application Scenarios and Recognition Methods
1. Application fields
Due to its excellent self-sealing performance, RC/PT threads are widely used in various fluid transmission pipelines,
especially in pneumatic systems, medium and low-pressure hydraulic systems, water pipes, gas pipes, and fire protection
pipes. Whether it's valves, connectors or sensors, wherever reliable threaded sealing is required, you can find their
presence.
2. How to identify?
In modern equipment or drawings, you are more likely to see the "Rc" mark (if it is an internal thread) or the "R" mark
(if it is an external thread) that conforms to the ISO standard. If you see the "PT" mark on an old Japanese equipment
or some drawings that follow the old standards, it can be generally determined that it is the RC thread we are talking
about. The most reliable method of identification is to use a 55° thread gauge for measurement. If it can be smoothly
matched, then its identity can be confirmed.
3. Distinction between G threads and NPT threads
In daily work, the three types of threads - RC, G, and NPT - are the most prone to confusion. Please remember a simple
mnemonic for the distinction: RC is a 55° tapered thread, G is a 55° straight thread (cylindrical thread),
and NPT is a 60° tapered thread. G threads require end-face sealing, while RC and NPT rely on the thread itself
for sealing. However, RC and NPT cannot be interchanged due to their different tooth profile angles.
IV. Conclusion: Different titles, but the essence is the same
Through the above analysis, we can draw a clear conclusion: The RC thread and the PT thread are completely
identical in terms of design standards, technical parameters and functions, and can be interchanged.
Their main differences lie in the historical nomenclature and the attribution of the standard system. PT is a product
of the old JIS standard, while RC is the standardized term for the current ISO international standard and the
Chinese national standard (GB/T 7306.2). In today's context of global production and trade, understanding and using
the RC standard nomenclature helps avoid misunderstandings in communication and ensures the accuracy
of procurement and assembly.
Here are the questions you might be interested in and their corresponding answers:
Q: Can RC threads and PT threads be used interchangeably?
A: Yes. Because their technical parameters, such as the tooth profile angle (55°) and taper (1:16), are exactly
the same, they are functionally equivalent and interchangeable. PT can be regarded as the old name for RC.
Q: How can one quickly distinguish between RC threads and NPT threads using the naked eye?
A: Although both are tapered threads, the top and bottom of the RC (PT) thread are arc-shaped,
giving it a more rounded appearance, while the top and bottom of the NPT thread are flat,
making it look more "sharp".
Q: Can G thread and RC thread be screwed together?
A: In certain cases, cylindrical internal threads (G) can be used in conjunction with conical external threads (R),
but this is not an optimal connection method. The sealing effect is not as good as that of "cone to cone" (R/Rc)
or "cone to cylinder" (R/Rp) combinations.
