- Huyett Marketing Department
- 01/27/2022
- Updated 02/22/2024
Pipe plugs are threaded plugs that seal the ends of pipes to prevent the flow of fluids. They are used in industries ranging from domestic plumbing and construction to hydraulics, gas and oil transportation, agriculture, food and beverage manufacturing, and industrial chemical processing. In short, wherever there are pipes, you’ll find pipe plugs.
As you might expect from such a common part, pipe plugs come in many different designs and sizes, which can make it challenging to select the best pipe plug for your application. That’s why we’re helping you narrow down your search from our 1,300 pipe plug SKUs to the perfect one for your application with this helpful guide. Let’s dive in with a brief overview of how this essential fastener works.
How Do Pipe Plugs Work?
Pipe plugs are cylindrical metal fittings with a taper-threaded shaft at one end and a drive at the other. The tapered threads have a slight cone profile, causing the male and female threads to compress against each other to form a tight seal. In use, the male threaded shaft is screwed into the female thread of a pipe or housing. The primary role of a pipe plug is to block the flow of fluids, but they are also used to prevent dirt and other contaminants from entering an opening in a pipe.
Before we get to the details, we should mention that the term “pipe plug” also refers to inflatable pneumatic pipe sealers used to block the flow of fluids in large pipelines, often for leak testing and maintenance. However, this article discusses threaded metal pipe plugs rather than inflatable or expandable pipe plugs.
Pipe Plugs vs. Pipe Caps
Pipe plugs have male threads and are screwed internally into the end of a pipe. In contrast, pipe caps have female threads and are screwed onto the outside of the pipe.
Pipe plugs and pipe caps perform the same function; however, pipe plugs can be used in locations where it’s not feasible to put a cap over a pipe’s end. For example, pipe plugs can be used to close an opening in the side of a larger pipe or the wall of a machine housing.
Pipe Plugs vs. Drain Plugs
Pipe plugs and drain plugs are similar products, but they differ in the specification of their threads. Pipe plugs have National Pipe Taper Fuel (NPTF) threads, whereas drain plugs have PTF-SAE Short threads, also known as PTF threads. Both threads are tapered to resist leaking; however, PTF-SAE threads have one full thread less than NPTF threads, allowing for a lower profile. Drain plugs are often used in applications where clearance of moving parts is a consideration.
When Should You Use A Threaded Pipe Plug?
Threaded pipe plugs are suitable for most applications in which plumbers need to end-stop a pipe. They are convenient, durable, and easy to install and remove with widely available tools. However, pipe plugs require an internally threaded pipe, which may not always be available.
Alternatives to threaded pipe plugs include threaded pipe caps, as we’ve already discussed. Where threads are not available, plumbers often solder unthreaded caps, particularly on domestic copper piping. In industrial plumbing scenarios with large pipes, butt welded or socket welded pipe caps may be used instead of threaded plugs. Correctly welded caps are very strong, but they are permanent and require special equipment to install.
How to Measure Pipe Fittings
Selecting a pipe plug of the correct size can be challenging because of inconsistencies in the pipe thread size designations and pipe plug sizes. It is not possible to find the correct pipe plug size by simply measuring the outside or inside diameter of the pipe and selecting a matching part.
To find the correct pipe plug for your pipe, follow these simple steps:
- Measure the pipe’s inside diameter.
- Use a thread chart to determine the thread’s nominal size.
- Measure the thread diameter and subtract 1/4 inch to find the nominal pipe size.
- Find the measured thread diameter on the chart and then the equivalent nominal size.
*This chart is available to download here:
National Pipe Tapered (NPT) Thread Chart
INCH SIZE
| DASH SIZE
| NOMINAL THREAD SIZE
| MALE THREAD
O.D. (in.) | FEMALE THREAD
I.D. (in.) |
|---|---|---|---|---|
1⁄
8 | 02
| 1/
8 - 27
| .39
| .33
|
1/
4 | 04
| 1/
4 - 18
| .52
| .44
|
3/
8 | 06
| 3/
8 - 18
| .65
| .58
|
1/
2 | 08
| 1/
2 - 14
| .81
| .70
|
3/
4 | 12
| 3/
4 - 14
| 1.02
| .92
|
1
| 16
| 1 - 11
1/
2 | 1.28
| 1.16
|
1
1/
4 | 20
| 1
1/
4 - 11
1/
2 | 1.62
| 1.50
|
1
1/
2 | 24
| 1
1/
2 - 11
1/
2 | 1.86
| 1.73
|
2
| 32
| 2 - 11
1/
2 | 2.33
| 2.22
|
How to Choose a Pipe Plug
There are six main factors to consider when choosing a pipe plug: the thread size, thread type, drive type, standard specifications, material, and finish. On Huyett pipe plug product pages, you will find descriptions such as:
Plug, Pipe 1/8‑27 NPTF, Hex Socket, SAE J531, Low Carbon Steel, Plain
This represents everything you need to know when selecting a pipe plug.
Thread Size and Thread Type
After the product type — pipe plug or drain plug, for example — you will see the thread size and type. In the example above, the thread size and type are "1/8‑27 NPTF."
- 1/8 is the thread size, as explained in the previous section.
- 27 indicates the thread pitch — the distance between crests. The thread pitch is determined by its size and type.
- NPTF indicates the thread type. The most common thread types are NPTF and PTF‑SAE, but we also stock British Standard Pipe Taper (BSPT) pipe plugs.
Reference the chart below for an explanation of the most common pipe thread types:
Common Thread Types
SAE-LT
(Society of Automotive Engineers) |  |
| Used on industrial machines and many other standard-duty applications
|
PTF
(American Standard Pipe Thread) |  |
| Creates seal by controlling truncation at crest and root of NPT form threads to allow for interference fit, closing pathway and preventing spiral leakage.
|
NPTF
(American Standard Pipe Thread Fuel) |  |
| Creates seal by controlling truncation at crest and root of NPT form threads to allow for interference fit, closing pathway and preventing spiral leakage.
|
UNF
(Unified National Fine Thread) |  |
| Common mechanical thread type used in the U.S. for screws and bolts
|
UNEF
(United National Extra Fine Thread) |  |
| Common mechanical thread type, similar to UNF, but with a finer thread count
|
ISO Metric
(Foreign Standard Pipe Thread) |  |
| One of the first international standards agreed upon when the International Organization for Standardization was created in 1947
|
BSPP
(British Standard Pipe Parallel) |  |
| Coarse threaded Whitworth for threads with a parallel thread profile
|
BSF
(British Standard Fine) |  |
| Parallel threaded Whitworth form threads
|
BSPT
(British Standard Pipe Tapered) |  |
| Whitworth form tapered male threads. Female threads are typically parallel.
|
German Thread
(British Standard Pipe Tapered) |  |
| Equivalent to BSPT in Germany
|
*This chart is available to download here:
Common Thread Types Chart
Pipe Plug Drive Types
Pipe plugs have a screw drive on the end opposite the threads. The drive mates with appropriately shaped tools to allow the plug to be screwed into position. Pipe plug screw drives are available in many types and sizes suitable for different tools and situations. In our example, the drive type is “
Hex Socket,” a six-point socket that mates with a tool fitted with a standard hex bit.
Other common drive types include:
Each of these is available in a range of thread sizes, allowing plumbers to select the thread size and drive type to suit their project.
Pipe Plug Standards
Following the drive type, you will see a standard designation. In the example, it’s “SAE J531.” SAE is the Society of Automotive Engineers—this pipe plug is commonly used in the automotive industry. Pipe plug standards define the plug’s specifications. SAE J531 defines dimensional specifications for square and hexagon pipe and drain plugs.
The specifications are helpful if you need a full breakdown of a pipe plug’s dimensions, including its body length, body width, head length, head height, and more.
Common pipe plug standards include:
SAE J531
Automotive Standard
This standard provides complete general and dimensional specifications for pipe, filler, and drain plugs commonly used in the automotive industry. Pipe plugs specified within this standard have NPTF threads. Filler and drain plugs within this standard have PTF-SAE Short threads.
ASME B16.11
Forged Fitting Standard
This standard covers the dimensions, ratings, tolerances, marking and material requirements for forged fittings. Pipe plug designs covered by this standard are the Hex and Square head types. Even though the standard identifies NPT threads, NPTF threads are applied as these threads provide better resistance to leakage over the NPT thread design. Where hot forged plugs are required, the governing ASTM standard must be stated.
ASME B16.14
Cast Fitting Standard
Provides technical information about plugs of certain designs manufactured from casting, ductile iron and steel. Even though the standard identifies NPT threads, NPTF threads are applied as these threads provide better resistance to leakage over the NPT thread design.
DIN 906/158
Hex Socket Metric Threads
This standard governs the manufacturing of hex socket pipe plugs with metric tapered pipe threads.
- Cold Formed
- Bright / Smooth Surface
DIN 909/158
Small Hex Head Metric Threads
This standard governs the manufacture of Small Hex Head pipe plugs with metric tapered pipe threads.
- Cold Formed
- Bright / Smooth Surface
DIN 906/3858
Hex Socket British Threads
This standard governs the manufacture of Hex Socket pipe plugs with British tapered pipe threads.
- Cold Formed
- Bright / Smooth Surface
DIN 909/3858
Small Hex Head British Threads
This standard governs the manufacture of Small Hex Head pipe plugs with British tapered pipe threads.
- Cold Formed
- Bright / Smooth Surface
For a more in-depth look at what fastener standards are and the organizations that provide them, check out our
Fastener Standards guide.
Pipe Plug Materials
Pipe plugs are manufactured in several types of metal because they are used in many different applications with pipes made of many different materials. When choosing a pipe plug, select one made of the same material as the pipe or one made of a material with desirable properties.
Commonly used materials include:
- Low carbon steel: This is hard and durable enough for most low-pressure applications.
- Stainless steel: A strong and corrosion-resistant metal comprising iron, carbon, and varying quantities of chromium.
- Alloy steels: Available in many different varieties. They are harder and more durable than low carbon steel.
- Aluminum: Ideal for use with aluminum pipes and where lower weights are a priority. However, uncoated aluminum may corrode more quickly in the presence of other metals, including iron and brass. It is also weaker than steel and should not be used in high-pressure environments.
- Brass: An alloy of zinc and copper. It is widely used in plumbing fasteners. Brass is malleable, highly resistant to corrosion, and suitable for moderately high temperature environments such as hot water pipes.
Pipe Plug Finishes
The final section of the product description is the finish, which adds a protective layer. Our example has a "Plain" finish, which means nothing has been added to the metal surface.
Other finish options include:
- Passivated: A finish used for stainless steel pipe plugs and drain plugs. The steel is exposed to an acid, which causes the formation of a corrosion resistant oxide layer.
- Black oxide: A conversion coating process that forms a corrosion-resistant coating of magnetite (Fe3O4), which may then be then impregnated with oil or wax.
- Zinc: A corrosion-resistant finish that often provides a clear, gold, black, or green color.
For a closer look at common finishing options, read our
Types of Metal Finishes for Fasteners introductory guide.
Common Pipe Plug Questions
In addition to learning the characteristics of pipe plugs, it is also important to understand the answers to these few common questions about these parts.
What is a Tapered Thread?
Tapered threads on a pipe plug will taper in slightly from just under the head of the plug to the end of the threaded shank. This is different than parallel threads, which form a straight line all the way from the under head to the end. The taper helps facilitate a stronger seal.
3/4" Taper Seal
3/4" taper is the standard taper for NPT pipe threads. Plugs with these threads are not designed to install flush to the surrounding surface.
7/8" Taper Seal
7/8" taper threads are unique in that they will allow the plug to install flush with the surrounding surface. These plugs can only be installed into tapered NPT or NPTF threaded holes.
Drain Plugs
In addition to pipe plugs, we offer drain plugs as well. Drain plugs differ from pipe plugs in that they feature PTF-SAE Short threads. This thread profile has one less thread than NPTF thread profile.
How to Install Metal Pipe Plugs
Installing metal pipe plugs is a straightforward process that only takes a few tools.
- Depending on the plug’s head or drive type, ensure you have the correct wrench or drive tool to install the plug.
- Before connecting the plug to the pipe, it is common practice to apply some sort of thread sealant (like specialized tape or thread compound) to fortify the connection.
- After applying the thread sealant, twist the plug into place and tighten it with the corresponding installation tool.
Where to Buy Pipe Plugs and Drain Plugs
Huyett stocks a comprehensive selection of pipe plugs in all sizes, thread types, drive types, materials, and finishes mentioned here, and many others we haven’t discussed. View our
full selection of pipe and drain plugs and
O-rings or contact our
knowledgeable and friendly Sales team.
