What are sealing plugs? A complete guide
Sealing plugs close off openings, cavities, or passages in different mechanical and structural systems. Their main job is to block out fluids, gases, and other unwanted materials, helping to protect the system and keep everything working as it should. To give you a better understanding of sealing plugs, we’ve put together this guide, covering:
How sealing plugs work: mechanism and functionality
Key applications of sealing plugs industries and use cases
How to choose the right sealing plug
Installation and maintenance of sealing plugs
How sealing plugs work: mechanism and functionality
Expansion sealing plugs work through a mechanical expansion process that creates a tight, permanent seal within a drilled hole or cavity. Here's how they operate step by step:
1. Insertion: The plug assembly—which typically includes a sleeve and a ball or tapered pin—is placed into a precisely drilled hole.
2. Expansion: The hard ball (or sometimes a pin) is driven or pressed into the plug sleeve. As it moves into the sleeve, it forces the plug to expand outward.
3. Sealing action: This outward expansion pushes grooved sealing rings or ridges on the plug against the walls of the hole. The grooves bite into the surrounding material (usually metal), ensuring a tight mechanical and Gas/fluid seal.
4. Retention: Once the ball is fully seated, it remains locked in place, maintaining constant outward pressure to keep the seal secure and stable over time.
Browse our range of metric and imperial sealing plugs
Don’t confuse sealing plugs with Blaning plugs, which are used to close off unused holes. Our experts explain more in our Guide to hole plugs.
Key applications of sealing plugs industries and use cases
Sealing plugs are a practical solution for systems that need reliable sealing and minimal maintenance. Below are some of the most common applications and industries where sealing plugs are regularly used:
● Pneumatics – For sealing air passages and maintaining consistent pressure in pneumatic tools and systems
● Hydraulics – Used in pumps, manifolds, and valves to securely seal high-pressure fluid channels
● Fluid power systems – Essential for managing and sealing circuits that use liquid or gas to transmit power
● Industrial machinery – Ideal for sealing access points, and ports in various types of equipment
● Aerospace – Trusted in Aerospace systems where lightweight, high-performance sealing is critical
● Valves and regulators – Close off test ports or auxiliary openings in pressure control devices
● Cylinders and actuators – Ensure airtight or fluid-tight performance in both pneumatic and hydraulic cylinders.
How to choose the right sealing plug
When selecting a sealing plug, it’s not enough to simply match the plug to the hole diameter. An equally important—yet often overlooked—consideration is the material around the hole: its wall thickness and its distance from any nearby edges. These factors play a crucial role in ensuring the plug stays secure and sealed over time.
Why it matters
Expansion sealing plugs work by pressing outward against the walls of the hole. This outward pressure deforms the surrounding material slightly and permanently (a process called plastic deformation), creating a tight, long-lasting seal. But if there isn’t enough solid material around the hole, the surrounding metal can weaken, crack, or distort, compromising the seal or even leading to plug failure.
The illustration here explains how to calculate the minimum required wall thickness or edge distance (w_min) when installing sealing plugs, depending on the diameter of the hole (d₁). It ensures the structural integrity of the material around the plug and helps prevent failure during plug expansion.
Key variables:
d₁: The diameter of the drilled hole for the sealing plug.
w_min: The minimum distance from the edge of the hole to the nearest external wall or adjacent hole.
f_min: A multiplier (provided by the plug manufacturer) used to determine safe edge clearance.
When d₁ ≥ 4 mm
w_min = f_min × d₁
● For holes 4 mm or larger, the minimum wall distance is a direct multiple of the hole diameter.
● Example: If d₁ = 5 mm and f_min = 0,8 then w_min = 4 mm.
When d₁ < 4 mm
w_min = (f_min × d₁) + 0.5 mm
● For small holes (< 4 mm), an extra 0.5 mm is added to ensure stability, because thinner walls are more prone to distortion or cracking.
Installation and maintenance of sealing plugs
Position the plug into the counterbored hole, ensuring the ball is oriented outward and rests against the shoulder of the counterbore.
Drive the ball into the plug sleeve until it sits just below the sleeve’s top surface (refer to the table below for approximate values of x and s).
Be sure to use the proper installation tool specific to the plug size. Installation can be done manually with a hammer and tool, or with an air hammer using the corresponding pneumatic installation accessory.
Tools
Installation tools are essential for ensuring sealing plugs are fitted securely and perform reliably.
Setting tools for sealing plugs
Designed to simplify the installation of sealing plugs and ensure a reliable, leak-proof seal. By pressing the expander ball into the plug sleeve, the grooved sealing rings are forced into the housing material to permanently close the hole—no threading, reaming, or O-ring groove machining needed.
These tools help reduce air and contaminant leakage and are available in both manual (hand) and pneumatic (air) variants to suit different installation needs. Common applications include pneumatics, hydraulics, fluid power systems, automotive, industrial machinery, and valves. Always ensure the ball is fully seated before applying system pressure.
Browse our range of metric setting tools for sealing plugs
Browse our range of imperial setting tools for sealing plugs
Download free CADs
Free CADs are available for most solutions, which you can download. If you’re not quite sure which sealing plug will work best for your application, our experts are always happy to advise you. Whatever your requirements, you can depend on fast dispatch.
You might find it helpful to also check out our Guide to high-temperature masking plugs and Tapered plugs vs parallel plugs.
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 at 0345 528 0474.