A Guide to Thread Protection Caps and Plugs

Yellow plugs at the end of pipes

Thread is vulnerable to damage during manufacturing, storage and transportation. This is why threaded components need the protection of threaded caps and plugs. In this guide, we’ll explain how to protect external and internal threads with plastic thread-protection caps and plugs, also available in metric threads. 

What we'll cover:

Threaded cap

What are threaded caps?

Thread-protection caps and plugs have distinctly different purposes. 

A threaded cap screws over an exposed threaded component on your application, as shown here. These caps, also known as thread protectors, serve a vital purpose: protect your component’s threads from damage, which can easily occur during mishandling at various stages, from manufacturing to transportation. 

They come in different thread sizes to ensure a precise fit. They’re available in metal, but mostly in different plastic materials. Plastic thread protector caps also form an airtight seal to protect against moisture and other damaging elements that can corrode the part.

Threaded plug

What are threaded plugs?

Where threaded sealing caps protect external threads, threaded hole plugs protect a component’s internal threads. Instead of fitting over the area, they fit into the component, as shown here. 

In this example, the threaded plug includes an optional o-ring. Some threaded protection plugs also come with nitrile gasket seals. O-rings and gasket seals provide a watertight grip and prevent the entry of contamination.

Both thread-protection caps and plugs are available in a range of different thread sizes. 

What are threaded caps used for?

Between 75% to 85% of all hydraulic hose failures are caused by contamination, such as dirt, dust and moisture, which in turn cause corrosion.1 Contaminants can enter and circulate within the system at any point – even before the part is installed on machines. An end cap with threads protects components from these threats. 

In addition to hydraulics, they’re also used to cap off tube and pipe ends. You’ll find them in automotive, pneumatic and engineering applications, providing protection to welding areas and studs for masking processes and preventing damage caused by mishandling. 

What are threaded plugs used for?

The same purpose that caps serve for hydraulic fittings, but protection plugs prevent dust, moisture, and dirt ingress – and the ensuing corrosion – from damaging internal threads. 

The highest contamination levels compared to other areas in the hydraulic system are usually found in reservoirs and plumbing. Like threaded screw caps, plastic plugs provide a quick fitting and removal to protect these systems and their threaded components. 

And when you want to prevent or limit fluid leakage, a threaded hole plug with a seal or o-ring is ideal due to its precise fit. 

Like caps, threaded plugs are used across multiple industries, including automotive, pneumatic and engineering. Applications also include masking processes to protect electrical contact points and internal threads. 

If you’re looking for reusable caps and plugs for masking, the material plays an important role. Threaded caps and plugs are typically manufactured in low-density polyethylene (LDPE) and high-density polyethylene (HDPE), just to give a few examples. Unthreaded silicone caps are your best bet for reusability. We suggest you learn more by checking out our Masking materials selection guide .

Standards for threaded caps and plugs

When choosing your thread-protection caps and plugs, be sure to get the relevant standard of thread that your component calls for. Your choices include:

Thread standard What it means
UNF Unified Fine Thread, mostly used in U.S. and Canada.
Metric Worldwide standard as per the ISO (International Organization for Standardization).
BSP/Gas Used on most UK oxygen, acetylene & propane gas equipment.
UJF Thread standard per ASME B1.15. Note, UNJ is not interchangeable with UNF. UNJ has a larger root radius than a standard thread pitch. 
UJFS UNJ threads are designated by the nomenclature UNJS.
NPT National Pipe Thread. U.S. standard for screw threads used on threaded pipes and pipe fittings.
JIC  Joint Industry Council; fittings that meet the Society of Automotive Engineers (SAE) standard.
NF-NS American National Standard; NF means fine thread and NS means special thread,


Black caps on large blue pipes

Types of thread-protection caps and plugs

What plastic threaded caps best suit your needs? Which plastic threaded plug? Types vary and offer different attributes. You’ll find it helpful to check out The ultimate guide to caps and plugs. Below are a variety of thread-protection caps and plugs, along with their thread standard. 

Threaded caps

BSP/Gas thread

BSP/Gas thread

Fewer thread forms and an easy-to-grip head allow for efficient application and removal. Protects against damage and contamination ingress, from production to storage. 

  • Red LDPE
  • Thread sizes & pitch:  ⅜" - 24 to 1 ⅞" - 12
  • Also available in metric thread.
  • Thread sizes & pitch: M8 x 1 to M52 x 2 
UNJ/UNJS thread

UNJ/UNJS thread

Head design enables easy grip and installation. Protects against damage and contamination ingress, from production to storage.

  • Black HDPE
  • Thread sizes & pitch: ⅜ - 32" to 1 37/64" - 14
UNF thread

UNF thread

Known as threaded sealing caps. Neoprene sealing disk limits fluid leakage

  • Yellow HDPE
  • Thread sizes & pitch: ⅜" - 24 to 1⅞" - 12
  • Also available in metric thread
  • Thread size & pitch: M8 x 1 to M52 x 2

Threaded plugs

UNF threads

UNF threads

Threaded O-ring plugs can be applied or removed by hand, hex wrench or screwdriver. The O-ring provides a watertight seal for superior leak protection.

  • Yellow polypropylene (PP)
  • Also available in: Natural glass-filled nylon and red 
  • Thread sizes & pitch: 7/16" - 20 to 1⅝" - 12
  • Available in metric thread
  • Thread sizes & pitch: M10 x 2 to M52 x 2   
NPT threads

NPT threads

Square head design enables installation and or removal by hand or wrench. Protects NPT threaded ports against contaminants and moisture.

  • Available in blue HDPE and natural nylon
  • Thread sizes & pitch: ⅛" - 27 to 2" -11 - 1/2    
BSP/Gas thread

BSP/Gas thread

Knurled head design for easy grip enables fast assembly or removal by hand. Secure fit provides excellent protection against dirt, moisture and corrosion.

  • Red LDPE
  • Thread sizes & pitch: ⅛" - 28 to 3" - 11
  • Also available in metric thread.
  • Thread sizes & pitch: M8 x 1 to M52 x 1.5    
NPT thread

NPT thread

Unique head design allows application or removal by hand or with a 12-point socket head, screwdriver or wrench. Protects NPT threaded ports. Optional o-ring provides a watertight seal to prevent leakage. 

  • Natural glass-filled nylon or black HDPE
  • Thread sizes & pitch: ⅛" x 1"
NS-NF thread

NS-NF thread

Apply or remove by hand, hex wrench or screwdriver. Fits NS and NF Class 1-2-3 standard threads. Optional o-ring provides a watertight seal to prevent leakage. 

  • Red Polypropylene (PP)
  • Thread sizes & pitch: 7/16" - 20 to 1" - 14
37° JIC flared fittings

37° JIC flared fittings

Knurled head for easy finger installation. Used to seal light pressure requirements on 37° JIC flared fittings. Functionally Equivalent to Spec: NAS-833, NAS-815 (MIL-C5501/1, MIL-C52078).

  • Red LDPE
  • Thread sizes & pitch: 5/16" - 24 to 2 1/2" - 12 
12 - 359224_1460x500px.jpg

UNJ/UNJS thread

Designed with an easy-to-grip head for quick hand installation. Protects threaded components from dust, moisture and damage during production, storage and shipment.

  • Red HDPE
  • Thread sizes & pitch: 5/16" - 24 to 2 1/2" - 12 
Quick fit – UNF, JIC, & Metric

Quick fit – UNF, JIC, & Metric

Fewer thread forms enable quick application.

  • Red LDPE
  • UNF & JIC thread sizes & pitch: 7/16" × 20 to 1 5/16" × 12
  • Metric thread sizes & pitch: M14 x 1.5 to M52 x 2

To see more thread-protection caps and plugs, view the entire range here

Which material should you use?

It would benefit you to read our Ultimate guide to caps and plugs. Here, we go in-depth into materials and their properties, and how similar materials differ from each other

Download free CADs and try before you buy

Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the solutions you’ve chosen are exactly what you need. If you’re not quite sure which threaded plastic caps and plugs will work best for your application, our experts are always happy to advise you. Whatever your requirements, you can depend on fast dispatch. Request your free samples or download free CADs now.


Email us at sales@essentracomponents.com or speak to one of our experts for further information on the ideal solution for your application 800-847-0486.