A guide to sealing screws and bolts

In pressure-rated or environmentally sealed systems, fastener leakage is one of the most common and overlooked failure points. Sealing fasteners solve this with purpose-built designs that combine mechanical strength with controlled sealing performance. This guide is written for engineers and technical professionals responsible for specifying fasteners in sealed systems. If you’re working on equipment exposed to pressure, washdowns, vibration, or contamination risks, sealing fasteners are not a detail. They’re a design-critical component. Here, we’ll cover:

What are sealing screws and bolts?
Types
Benefits of choosing captive O-ring screws
Applications and industries
Sealing fastener sizes: What you need to know

What are sealing screws and bolts?

Sealing screws and sealing bolts are fasteners designed to create a secure mechanical connection while also preventing the passage of fluids, gases, or contaminants through the fastening point. They use an O-ring, typically seated in a groove beneath the head or around the threads, that compresses during tightening to seal the joint and block fluid or contaminant ingress.

Sealing screws vs sealing bolts

While both provide sealing, screws and bolts are mechanically different. Screws are used where space is limited and loads are modest. They’re installed with a driver, typically by hand or in low-torque assembly setups.

Bolts are used where higher clamping force is needed. They’re torqued with a wrench or socket and suited to structural joints, heavy equipment, or assemblies subject to vibration or load cycling.

Screws, on the other hand, are more common in smaller assemblies or electronics. Unlike standard fasteners, true seal screws are specifically built for sealing performance. 

Note, sealing fasteners are engineered systems, not modified hardware. Simply adding an O-ring to a standard fastener will not provide the same protection and may compromise the integrity of the seal.

To help you learn about these fastener types, our experts explain what you need to know in Bolts vs. screws: What’s the difference and when to use each.

Types

Sealing stainless steel screws and bolts come in various forms designed to suit different mechanical and environmental needs. Though they all share the same core purpose, providing a secure connection while preventing leaks, they differ in structure and performance characteristics. 

Shared features

●    Reliable sealing: O-ring compresses between the head and mating surface, forming a watertight, airtight, and dust-tight seal.
●    Durable construction: Available in 303 and 316 stainless steel for excellent corrosion resistance and long service life.
●    Versatile applications: Suitable for both threaded and clearance holes. For optimal results, clearance holes should be 2–3× the thread pitch and use metric coarse threads.
●    Resilient under movement: Maintains sealing performance even under vibration, thermal expansion, or mechanical stress.

Chemical & environmental resistance  

Our sealing screws and bolts are supplied with silicone O-rings as standard. Silicone provides stable sealing performance across a wide temperature range and is valued for its resistance to weathering and ozone.

Highly resistant to:

●    Water and steam, including continuous hot water and pressurised steam
●    Ozone and UV light, excellent for outdoor and long-term use
●    Mild acids such as acetic acid (vinegar), citric acid, boric acid
●    Mild alkalis such as dilute ammonia solution or sodium carbonate (washing soda)
●    Operating temperatures from −60 °C to +200 °C

Less suitable for:
●    Petroleum-based fluids: mineral oils, fuels, diesel, kerosene, lubricants
●    Strong acids: concentrated sulfuric acid, hydrochloric acid, nitric acid
●    Strong alkalis: concentrated sodium hydroxide, potassium hydroxide
●    Organic solvents: aromatic hydrocarbons (toluene, benzene), ketones (acetone, MEK), and esters

Clearance holes for optimal sealing

For best results, sealing screws should be installed in clearance holes rather than threaded holes. A clearance hole is an unthreaded hole, slightly larger than the screw diameter, which allows the screw to pass through freely. This ensures the O-ring under the head can compress evenly against the surface and form a reliable seal.

Recommended setup:
●    Clearance hole diameter: 2–3× the thread pitch
●    Thread type: Metric coarse
 

Option 2: Clearance holes in both components

Here, both components have clearance holes. The screw passes through both parts, and the seal is achieved under the head against the top surface. The fastening method beyond the clearance holes (e.g. into a backing plate, captive insert, or nut) depends on the assembly design, but the sealing principle remains the same: the O-ring compresses between the head and the outer surface.

You might also like Sealing Screws and Sealing Nuts From Wixroyd.

Benefits of choosing captive O-ring screws

The term captive O-ring screws refers to sealing screws with an O-ring that is permanently retained in a groove under the head. This ensures the O-ring stays in place during handling and installation, eliminating the need to fit separate seals.

Category	Benefits
Superior sealing	– Bi-directional seal prevents both internal leakage and external ingress. – Protects sensitive components from contamination or hazardous leaks. – Controlled O-ring compression provides consistent long-term performance.
Cost & efficiency	– Speeds up assembly — no manual O-ring fitting or external sealants required. – Reusable design thanks to controlled compression. – Removes the need for additional gaskets or washers. – Longer service life reduces maintenance and replacement costs.
Durability & Reliability	– Performs reliably in harsh environments: temperature extremes, chemical exposure, vibration, and UV. – Maintains structural integrity through metal-to-metal load transfer.

Applications and industries

Sealing fasteners are especially valuable in applications that involve washdowns or sensitive electronics. They can also help ensure compliance with IP and industry-specific sealing standards.

Application / Industry	Purpose & benefits of sealing fasteners	Choose
Machine builders	Used in custom or automated machinery to protect internal mechanisms from dust, oil, & moisture. Improve safety, uptime, & equipment longevity.	Button head sealing screws Cap head sealing screws Integral seal bolts
Electrical enclosures	Maintain watertight & dustproof seals in control panels & cabinets, particularly in outdoor or industrial settings.	Pan head sealing screws Integral seal bolts Anu-Lok sealing screws
Automotive systems	Used in body components, engine compartments, & sensor housings where vibration resistance & leak prevention are critical.	Cap head sealing screws Anu-Lok sealing screws Integral seal bolts
Valves, pumps & piping systems	Prevent fluid leaks & maintain pressure in systems transporting liquids or gases. O-ring compression ensures long-term sealing even under thermal expansion or vibration.	Integral seal bolts
Electronics & PCBs	Protect sensitive electronic assemblies from humidity, dust, & EMI/RFI interference. Often required in mission-critical or harsh environments.	Pan head sealing screws Anu-Lok sealing screws Button head sealing screws
Medical & cleanroom equipment	Provide reliable, contaminant-free seals to meet strict hygiene or sterilisation requirements, preventing foreign object debris (FOD) risks.	Anu-Lok sealing screws Cap head sealing costs
Aerospace & defense	Help avoid FOD, ensure fastener retention, & maintain sealing under extreme pressure, altitude, or temperature variations.	Anu-Lok sealing screws Integral seal bolts
Marine & outdoor equipment	Offer corrosion resistance & tight sealing in environments exposed to saltwater, rain, and UV exposure.	Integral seal bolts Button head sealing screws Pan head sealing screws

 

Sealing fastener sizes: What you need to know

Size selection isn’t just about fit. It affects sealing performance, thread engagement, and mechanical stability. Below is a breakdown to guide proper specification. 

Thread size range: Overview  

Most sealing fasteners from Essentra are available in the size you need. 

Fastener type	Thread size range
Pan head	M2 – M8
Cap head	M3 – M8
Button head	M3 – M12
Anu‑Lok cap head	M3 – M16
Integral seal bolts	M2 – M12

 

We recommend that you also read our guide, How to measure bolt size.

Dimensional profiles by head style

Each fastener type has different head dimensions (height, diameter), which may impact clearance or tool access.

Head type	Profile	Use case example
Pan head	Low profile, wide head	Covers larger surface area, low clearance
Cap head	Tall, cylindrical head	More torque and deeper socket drive
Button head	Rounded, aesthetic low-rise	Visible panels or compact builds
Anu‑Lok cap head	Cap head with nylon patch	Anti-vibration and sealing in one
Integral seal bolts	Hex head with underhead O-ring groove	Robust sealing for machinery

Suggestion:

If your application involves tight space or cosmetic requirements, pan head or button head screws may be ideal. For torque and strength, cap head or integral seal-bolt styles are better suited.

How long should your sealing screw be?

We recommend that you:

1.    Measure the total thickness of all components being fastened.
2.    Add at least 1× thread diameter in depth for proper engagement.

For example, If you’re fastening a 5 mm plastic panel with an M4 screw, use at least 8–10 mm length to ensure secure threading into a metal insert or nut.

For through-holes with washers and spacers, lengths of 16–30 mm are typical, 
depending on stack-up.

Drive types: Which one suits your tools or assembly line?

As a rule of thumb, keep this advice in mind:
●    Torx is preferred for automation and high-volume assembly: reduced cam-out and excellent torque transfer.
●    Phillips and Slotted are common but may cam out at higher torque. These drive types are better suited for manual fastening.
●    Hex Socket (Allen) offers clean looks and good torque.
●    External Hex (on integral seal bolts) is perfect for spanner/wrench-based installs.

Our experts explain more in our guide, What are screws heads, drives and threads?

Download free CADs

Free CADs are available for most solutions, which you can download. If you’re not quite sure which sealing fastener will work best for your application, our experts are always happy to advise you. Whatever your requirements, you can depend on fast dispatch. 

Download free CADs

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.