Do you recognize all the different types of pipe threads? Analysis of the 3 most common pipe thread depths.

Pipe threads are specifically designed to "lock pipes together" and are used in pipeline systems that transport various fluids such as water, oil, air, and steam, including compressed air pipelines, hydraulic oil pipelines, water pipes, and gas pipes. Unlike general mechanical bolt threads, the design focus of pipe threads is on "sealing" and "assembly space," rather than simply tensile or clamping force.

A common point of confusion is that the dimensions indicated on pipe threads are usually "nominal diameters," which correspond to an approximate inner diameter or flow capacity of the pipe, not the actual measured outer diameter. For example, a 1/2″ pipe thread has an actual outer diameter of approximately 20.9 mm, but looks more like a 3/4″ thread. Measuring the outer diameter with calipers alone can be very confusing. Therefore, when designing or selecting a pipe thread, it is essential to refer to standard tables or catalogs, rather than relying solely on measured dimensions.

Straight threads vs. tapered threads: completely different sealing methods

Pipe threads can be broadly classified into two categories: straight threads (parallel threads) and tapered threads (conical threads). They differ not only in appearance, but also in tooth angle, sealing principle, and usage.

Straight pipe threads have a cylindrical tooth profile with almost parallel sides, and a common tooth angle is 55° (such as BSPP). This type of thread does not provide a seal; its primary function is "positioning and locking." True sealing relies on components such as flat gaskets, O-rings, or metal conical surfaces. For example, a flat sealing surface is created at the bottom of the joint, which is used in conjunction with a copper or rubber gasket. The advantages are repeated assembly and disassembly, precise positioning, and no risk of the thread "digging deeper" and changing direction with repeated tightening. The disadvantages are a larger number of parts, and care must be taken during assembly to ensure that the gaskets are fully compressed.

Tapered pipe threads are conical in shape, with a common thread angle of 60° (such as NPT, PT, etc.). The thread body itself is the sealing surface. When the male and female threads are tightened, the conical surfaces press against each other, generating circumferential contact pressure. Combined with a small amount of sealing tape or thread sealant, excellent sealing performance can be achieved. Because the seal relies on "interference fit," torque control during assembly is crucial. Too loose and it will leak; too tight and it may cause the joint to crack or the pipe end to deform.

In terms of design selection, general equipment joints (such as compressors, pneumatic cylinders, and hydraulic valves) often use straight threads with O-ring seals on the body side, while tapered threads are mostly used at the on-site pipeline construction end, which facilitates on-site construction with PTFE tape or thread sealant.

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Types of pipe threads

Pipe threads mainly come in the following types:

British Standard Straight Thread BSPP=BSP=PF=G

The British Standard PF (Power Thread) is a parallel thread with a 55-degree thread angle. PF threads have relatively poor sealing properties and typically require gaskets or sealants to prevent leaks. PF threads are most common in Taiwan, primarily used for pipe connections such as water and gas pipes. A rough size table for the thread is as follows:

American standard tapered thread NPT = NPTF

American Standard Tapered Thread (NPT) is a tapered thread with a 60-degree thread angle. NPT threads offer good sealing performance and do not require gaskets or sealants. NPT threads are most common in North America and are primarily used for pipe connections such as oil and gas lines.

In NPT tapered pipe threads, the taper is 1:16, meaning that for every 16 length units, the diameter decreases by 1 unit. The taper is defined as "diameter change / length". A rough dimension table for the thread is as follows:

British Standard Tapered Thread BSPT=PT=R=RC

British Standard Tapered Thread (RC) is a tapered thread with a tooth angle of 55 degrees. The difference between RC and NPT threads is that RC threads have a sharper tooth tip, resulting in better sealing. RC threads are most common in Europe and Commonwealth countries, primarily used for pipeline connections such as oil pipes and gas pipes.

Practical Selection, Processing, and Installation Precautions

When designing actual machine tools or pipelines, the selection of pipe threads can be considered from several aspects: pressure rating, whether frequent disassembly and assembly are required, on-site construction habits, material properties, and corrosivity. High-pressure hydraulic systems often prefer O-ring straight threads for metal pipe fittings to avoid cracking of tapered threads under high torque; while low-pressure gas systems and cooling water pipes often use tapered threads with PTFE tape, which is quick to install and low in cost.

In terms of machining, the tapping depth and pilot hole size must comply with the manufacturer's recommended values and the standard table. Avoid tapping too shallow, resulting in insufficient effective thread length, or tapping too deep, causing the joint to be locked to the bottom without the expected interference. Inspection can be performed using a plug gauge (thread gauge) to confirm that the number of turns screwed in matches the specification, rather than relying solely on visual inspection.

During installation, for tapered threads, first remove oil and metal shavings, then wrap an appropriate amount of PTFE tape or apply threadlocker. When tightening, follow the principle of "turn by hand first, then use a wrench." Do not force it at the beginning, and do not apply excessive force to "turn it half a turn more to align the direction." For straight threads, ensure that the gasket or O-ring is not twisted or cut, and apply a little lubricant to the sealing surface to avoid dry friction scratching the sealing surface.

If you understand the principles, you won't be afraid of leaks in the piping.

In summary, while pipe threads may seem like just "teeth for locking pipes," they involve sealing mechanisms, dimensional marking conventions, standard systems, and on-site construction practices. Understanding the differences between straight and tapered threads, the meaning of imperial and metric markings, and paying attention to taper, effective thread length, and sealing methods can significantly reduce the risks of pipe leaks, frequent maintenance, and installation errors.

When processing drawings, selecting fittings, or installing piping on-site, it's helpful to ask questions like, "Is this a straight thread or a tapered thread?", "Where is the seal located?", and "Is the size DN or inches?" These simple confirmations will make your piping system much more stable and allow you to communicate more effectively with on-site technicians.