A guide to thread protection caps and plugs
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 threaded plastic caps and plugs. We'll cover:
What are threaded caps?
Threaded 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 threaded plastic caps, serve a vital purpose: to 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. Threaded caps made in different plastic materials are especially popular. Threaded plastic caps also form an airtight seal to protect against moisture and other damaging elements that can corrode the part.
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. A threaded cap 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.
To help you learn more, our experts have put together A guide to plastic end caps.
What are threaded plugs?
These are plugs with threads along its outer surface. The threads match the threads of a specific type of hole. The plug fits into a component. By twisting the plug, the threads interlock, creating a tight seal and plugging the opening.
Threaded plastic plugs can also include an optional o-ring. Some come with nitrile gasket seals. O-rings and gasket seals provide a watertight grip and prevent the entry of contamination.
What are threaded plugs used for?
The same purpose that caps serve for hydraulic fittings, but threaded 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, threaded 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 plug with a seal or o-ring is ideal due to its precise fit. Threaded caps and plugs with a seal or o-ring can withstand light pressure build up in the system, which is ideal for testing hydraulic lines.
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.
To learn about other types of plugs – some of which are threaded – in our guide, Hole plugs, explained.
Types of thread-protection caps and plugs
What threaded plastic 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
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: M8 x 1 to M52 x 2
- Also available in UNF thread.
- Thread sizes & pitch: ⅜" - 24 to 1 ⅞" - 12
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
Knurled head design makes it easy to install and remove by hand. Protects pipes and tubes against damage and contamination ingress, from production to storage.
- Red, yellow HDPE or LDPE
- Thread sizes: 1/8 to 2 ½
Threaded plugs
Threaded O-ring plugs with compressed flange lip provide a watertight seal for superior leak protection. Can be installed by hand, spanner, or screwdriver.
- Yellow HDPE
- Thread sizes & pitch: M10 x 2 to M52 x 2
- Available in UNF thread – Thread sizes & pitch: 7/16" – 24 to 17/8" – 12
Ideal for applications exposed to high fluid, flow residue and vibration. Knurled head design for easy grip. Secure fit provides excellent protection against dirt, moisture and corrosion.
- Red LDPE
- Thread sizes & pitch: M8 x 1 to M52 x 1.5 ⅛"
Snap Fit – Metric, BSP, UNF, JIC thread
Protects against ingress of contaminants. Easy to install and remove – simply push into position. A single twist provides a tight, snug fit, and a central tab at the top provides a strong grip for tightening and removal.
- Red LDPE
- Thread sizes & pitch: M12, M13, M14, 1/2"UNF, 1/2"JIC, 1/4"BSP to M40, M42, 1-5/8" UNF & JIC,1 -1/4"BSP
Fewer thread forms enable quick application.
- Red LDPE
- Metric thread sizes & pitch: M14 x 1.5 to M52 x 2
- UNF & JIC thread sizes & pitch: 7/16" × 20 to 1 5/16" × 12
Slottex Plugs – Metric, BSP & UNF
Designed with a double sealing ring on the underside of the flange to provide a secure and reliable seal. The design includes five complete threads for a secure fit.
- Nylon, HDPE
- Metric thread sizes & pitch: M8 x 1 to M52 x 2
- BSP thread sizes & pitch: ⅛” x 28 to 2” x 11
- UNF thread sizes & pitch: ⅜” x 24 to 1 ⅞” x 12
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
Protects threads and fittings from damage during production, storage, and shipment. Nitrile gasket seals the plug to the mating surface to prevent leaks.
- Yellow nylon
- Thread sizes & pitch: 7/16" x 20 to 1 7/8" x 12
Protects against damage and contaminants. Features fewer thread forms for faster application. Can apply or remove by hand, hex spanner, or screwdriver.
- PP, LDPE, HDPE
- Thread sizes & pitch: ⅜” in x 24 to 2 ½” in x 12
Standards for threaded caps and plugs
When choosing your threaded plastic caps and plugs, be sure to get the relevant standard of thread that your component calls for. Your choices include:
UNF
Unified Fine Thread, mostly used in the U.S. and Canada. Compared to standard threads, UNF (Unified National Fine) threads have a finer pitch. This means there are more threads packed into the same length, resulting in a tighter fit. This tighter fit translates to increased resistance to loosening due to vibration, making UNF threads ideal for applications where a strong connection and minimal movement are critical.
Metric
Worldwide standard as per the ISO (International Organisation for Standardisation). Thread sizes are designated in millimetres (mm) with an "M" followed by a number, such as "M8." This number directly tells you the nominal diameter of the thread in millimetres.
BSP/Gas
BSP stands for British Standard Pipe and is used on most UK oxygen, acetylene & propane gas equipment. These threads excel at creating leak-proof seals for fluids such as water and gas. Unlike some tapered thread designs, BSP/BSPP threads have a constant diameter along their length for a smooth fit. Sizes are designated by a nominal diameter (e.g., 1/2 inch BSP) but do not reflect the actual measurement.
UNJ
Thread standard per ASME B1.15. UNJ is designed for high-stress applications that need exceptional fatigue strength. Based on a military specification for aerospace fasteners, UNJ threads resemble standard UN threads but with a crucial difference: a larger, controlled radius at the root of the external thread. This design distributes stress more evenly, reducing the risk of cracks and improving overall strength. UNJ threads are ideal for critical applications such as high-performance machinery, and other demanding environments due to their superior strength and fatigue resistance.
UNJS
UNJ and UNJS threads are both specialised thread types designed for high-stress applications, but there are some key differences between them:
- Root radius: This is the main distinction. UNJ threads have a larger, semi-circular root radius compared to the standard V-shaped root of a UN thread. UNJS threads take this a step further with a controlled root radius. This means the shape is specifically designed and slightly more rounded than even a UNJ thread.
- Strength: The controlled root radius in UNJS threads offers a clear advantage. It improves the tensile stress area of the fastener and reduces stress concentration to a greater degree than UNJ threads. This translates to a stronger thread overall compared to UNJ.
NPT
NPT, which stands for National Pipe Thread, is a U.S. standard for tapered threads used to create leak-proof seals between pipes and fittings. It's commonly used in plumbing, hydraulics, and various industrial applications. NPT threads feature a 60-degree angle on the thread flanks with flat crests and roots. The key characteristic is their taper (¾ inch per foot), which creates a tighter seal as the pipe or fitting is screwed in.
JIC
Joint Industry Council; fittings that meet the Society of Automotive Engineers (SAE) standard. The JIC standard thread, also called SAE 37° flare, is a high-pressure fitting solution for fluids. It uses a 37° cone-shaped flare on the male fitting to create a metal-to-metal seal against a matching concave seat in the female fitting. This design is ideal for applications like hydraulic systems, fuel delivery in high-performance engines, and industrial machinery dealing with fluids, differentiating itself from NPT threads, which rely on a tapered thread for sealing.
NF-NS
NF stands for National Fine, a common thread type with a V-shaped profile and specific thread pitch. It's part of the Unified National (UN) system and widely used. NS, on the other hand, refers to National Special, an obsolete standard. NS threads were similar to NF but with non-standard dimensions. In 1955, the Unified National Special (UNS) system replaced NS, offering a more modern solution for applications where standard NF threads wouldn't suffice.
How to choose your threaded caps and plugs
Finding the ideal threaded caps and plugs for your project involves considering several key factors:
Environmental conditions
What kind of environment will your threaded plugs and caps be used in? This includes temperature range and exposure to chemicals, dirt, debris, moisture, liquids, and pressure (air or liquid). Will the cap be exposed to vibration?
Application
What's the job that your threaded caps and plugs need to perform? Are they for general protection or creating a seal? For general protection, LDPE caps are an excellent choice. For sealing, plugs with an o-ring will create a watertight seal.
Thread type
Ensure the cap or plug matches the specific thread type of the component you're working with (e.g. BSP/Gas, UNF, NPT, etc.)
Pressure rating
If relevant, the chosen cap or plug should be able to handle the expected pressure within the system.
Material selection
The material of your threaded plugs and caps needs to withstand the environment and any chemicals it will encounter. We’ll take a closer look at that next.
Installation and removal
Think about how you'll put the caps and plugs on and take them off. Some threaded caps and plugs can be applied by hand or with the use of tools. The fewer threads, the faster the application and removal.
We urge you to learn more in our guide, How to choose caps and plugs.
Materials
Choosing the right material for your threaded caps and plugs is essential for optimal performance. This table provides a quick reference guide to commonly used materials.
| Material | Advantages | Limitations | Best for |
|---|---|---|---|
| Low-density polyethylene (LDPE) | Affordable, good chemical resistance, flexible | Lower melting point (typically around 100°C) Lower mechanical strength |
Low-pressure applications (generally under 100 psi) General protection |
| High-density polyethylene (HDPE) | More durable and rigid than LDPE, good chemical resistance, handles slightly higher temperatures | Less flexible Lower melting point than nylon (around 130°C) |
Moderate-pressure applications (generally up to 200 psi) needing more mechanical strength |
| Polypropylene (PP) | Versatile, good chemical resistance, moderate rigidity, higher melting point than LDPE/HDPE | Brittle at very low temperatures (-20°C) | Moderate-temperature applications (generally up to 250 psi) Good balance of strength and cost |
| Glass-filled nylon | Exceptional strength, rigidity, dimensional stability, high melting point (around 260°C) | Most expensive, difficult to machine | Demanding applications requiring high strength and secure fit |
| Nylon | Good chemical resistance, strength, higher melting point (around 215°C) than LDPE/HDPE/PP | Not as strong as glass-filled nylon, brittle at very low temperatures | Moderate-temperature applications needing a balance of durability and cost |
Uses and functions
Threaded caps and plugs have a wide range of applications across various industries. Here's a breakdown based on their key functions:
Protection:
General protection: These are the most common uses. Threaded caps and plugs protect threaded components from dust, dirt, moisture, corrosion, and physical damage during various stages like:
- Production: Used to keep contaminants out of threaded components during manufacturing processes.
- Storage: They prevent dust, moisture, and other elements from accumulating on threads during storage.
- Shipment: Protect components from damage during transportation.
Specific protection:
- NPT threads: These caps and plugs specifically protect National Pipe Thread (NPT) threaded ports on pipes and fittings from contaminants and moisture.
- 37° JIC flared fittings: These caps seal light pressure requirements on specific flared fittings used in hydraulic systems.
- Closing unused ports: Threaded caps and plugs can be used to temporarily or permanently seal unused ports on equipment, preventing accidental leaks or contamination.
Sealing:
- Fluid Leak Prevention: Certain threaded caps and plugs, particularly those with sealing features like O-rings, are designed to prevent fluid leaks from threaded ports. These are commonly used in hydraulic systems and engine components.
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.
Questions?
Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 728 0474.