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Plugs, Stoppers, and Inserts Information

Threaded plug via Altech Corp. Plugs, stoppers, and inserts fit into holes or cavities and are designed for the ends of shafts, tubes, pipes, fittings, connectors, screws, bars, or other shapes. They are designed to fit inside a hole to block or protect something, compared to Caps, Grips, and Closures which fit on the outside of the component to be protected.

The type of the plug, stopper or insert is defined by the shape of the product or the means by which it is fitted. Plugs are available in many different shapes, materials, and a variety of different cross sections. These include round, button plug, square, rectangular, and screw plug.

Shape Types




Round-shaped Round or circular shaped plugs and stoppers have circular dimensions and are meant to fit onto objects with round ends, such as bottles or circular tubes.

Selecting round plugs

Image Credit: BMP Tappi

Square-shaped Square shaped plugs and inserts have equal length and width dimensions and are meant to fit onto objects with square ends.

Selecting square plugs

Image Credit: Enco

Rectangle-shaped Rectangular shaped plugs and stoppers have varying length and width dimensions and are meant to fit onto objects with rectangular ends.

Selecting rectagle plugs

Image Credit: Prescott Supply Company

Connection Type

Plugs, grips, and closures are available in several connection methods.




Threaded Threaded plugs have threads designed to attach to threaded ends of pipes, tubes, and other forms. Caps have female threads and plugs have male threads. Some threaded caps may incorporate a quick-twist style with a clip or lock that engages the cap in place.

Selecting threaded plugs

Image credit:Alliance Express

Tapered Tapered plugs, stoppers, and inserts are connectors with tapered sides, providing a snug fit. Some can be used as either a cap or a plug. Tapered plugs are versatile and can be used for straight tubes and bars, male and female NPT threads, and male straight threads.

Selecting tapered plugs

Image credit:Alliance Express

Expanding Tube Expanding tube plugs are used in moderate pressure applications, such as condensers and heat exchanger tubes. The plug designs usually consist of a central arbor which is pulled back into the body of the plug by tensioning a bolt. The bolt expands the body to lock the plug in place. Plugs designed for low pressure use often consist of a flexible body which is extended into the tube. This effectively reduces the diameter of the plug and allows it to slip into the tube. When the tool used for insertion is removed, the plug body returns to its original diameter and locks into the tube. Expanding and hammer-in plug types are fairly easy to remove should the tube be returned to service after repairs.

Selecting expanding plugs

Image credit: Conoco Systems

Inflatable Inflatable plugs are inserted into a pipe, tube, or other hollow object and then inflated with air to completely seal the hole. Inflatable plugs can be used to seal a range of hole sizes.

Selecting Inflatable Plugs

Image credit: Petersen Products

Sizing Specifications

Plug dimensions must be matched and selected according to the object they are covering or protecting. Sizing specifications will vary based on the shape of the plug.

Round Plug Specifications

When searching for plugs, stoppers, and inserts include the NPT size, BSP size, diameter, and round plug features.

  • Diameter is a dimension expressing the cross sectional distance spanning across the plug circumference. Plugs should be sized in order to fit the diameter of the round objects they are capping or covering.
  • BSP sizing applies to plugs designed to fit objects with male BSP (British Standard Pipe) threading.
  • NPT sizing is used for closures designed to plug objects with male NPT (National Pipe Thread) threads. Sizes range from 1/8NPT to 6+NPT.

Square or Rectangular Plug Specifications

These dimensions only apply to square or rectangular plugs.

  • Length is the larger cross sectional dimension for rectangular plugs. If the section is a square, the width and length represent the same dimension.
  • Width is the smaller cross sectional dimension for rectangular plugs. If the section is a square, the width and length represent the same dimension.


It is also important to consider the material of the cap, grip, or closure. Material typically dictates the physical properties of the cap, most importantly its hardness, flexibility, and resistance to wear or degradation. Additionally, it is often important for the material to match that of the item being protected or covered.


Stainless steel threaded bung plugAluminum is a very versatile metal that is lightweight, strong, durable, flexible, impermeable, corrosion-resistant, and recyclable.

Stainless steel is a steel alloy containing over 10.5% chromium. It is used in applications where excellent corrosion resistance and high strength are required.


Ethyl vinyl acetate (EVA) is a transparent thermoplastic elastomer. It remains flexible at low temperatures and resists cracking. Brand names include Ultrathene® (Quantum), Elvax® (DuPont), Evalca® (Eval).

Nylon, comprising several grades of polyamides, is a general purpose material in wide use; it is tough and resistant and has good pressure ratings and mechanical strength for its cost.

Polyethylene (PE) comes in high density (HD) and low density (LD) forms. The high density form is a high strength, rigid plastic with temperature resistance up to 215°F. The low density form is a flexible, semi-rigid plastic with a temperature resistance up to 175°F. Low density polyethylene is the most common material used for manufacturing caps and plugs.

Polypropylene (PP) is a high strength, rigid thermoplastic material exhibiting excellent cold flow, bi-axial strength, and yield elongation properties. It is similar to PVC but can be used in exposed applications because of its resistance to UV, weathering, and ozone. It resists temperatures up to 250°F.

Vinyl exhibits good flexibility and chemical resistance; it is used in many medical and chemical applications. Its properties can be enhanced or modified by compounding it with additives or modifiers to meet different application needs.

Other Material Types

Selecting Paper PlugsPaper caps are considered more environmentally friendly than plastics and metals and are typically 100% recyclable. They are frequently used for high-temperature applications that may degrade plastic caps or plugs. Because paper is hygroscopic, excessive humidity changes or water contact can cause dimensional variation or warping.

Silicone rubber is made from cross-linked silicone based polymers reinforced with filler. Silicone rubber is very resilient, and boasts long service life through adverse environments. The operating temperature range for silicone rubber is wide, from -148°F to 600°F.


Tapered plug via Ark - Plas ProductsAdditional features to consider for plugs, stoppers, and inserts include:

  • Flange - These plugs offer a flange for easy removal or additional protection of bearing surfaces. They also prevent accidentally pushing the plug all of the way through the opening.
  • Knurled or faceted head - Usually found on threaded plugs only, knurls or facets allow easier installation and removal, either by finger or wrench.
  • Slotted head - Slotted plugs can be installed or removed with a screwdriver.
  • Recessed socket - These plugs have a recessed socket for assembly with a wrench.
  • Lift or pull tabs - These tabs allow for easy removal of the plug. Plug has a tab or ring for easy removal or it is of pull plug construction. Pull plugs are designed for "through-hole" applications. They use a smaller diameter "pull section" to pull the plug until it is snug in the hole.
  • Ribs - Ribs are found on the mating part of the plug and help to provide a secure fit inside a tube. Ribs are available on round, square, and rectangular plugs.
  • Vented - Vented plugs are commonly used in the medical industry. The vent allows air flow during gas sterilization and prevents the cap from blowing off during extreme heat conditions.

Image credits:

Altech Corp | Ark - Plas Products | LADD Industries, Inc. | Estco Enterprises


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