Washer Seals

What are washer seals?

Washer seals, also called bonded sealing washers or bonded seals, are flat metal washers with a rubber sealing ring bonded to the inside. They fit under bolt heads, plugs, and fittings to stop fluid or gas leaking past the thread. When the bolt is tightened, the rubber is squeezed between the metal faces and creates a reliable high-pressure seal, while the metal ring stops it being over-compressed.

A standard washer seal has three main parts:

• An outer metal washer, usually mild steel, which takes the clamping load
• A rubber sealing ring, bonded to the inside of the washer, which is slightly wider than the metal
• A bore sized to fit a particular bolt or thread, such as a metric or BSP size.

The bonded rubber ring does the sealing.

• As the bolt tightens, the rubber compresses until the bolt head or fitting meets the metal washer, which stops it being squeezed too far.
• This gives a controlled, repeatable seal every time, without needing to judge the tightening exactly.

Washer seals are available in nitrile rubber as standard, with FKM for higher temperatures and harsher fluids, and other materials such as EPDM and silicone available on request.

Why are washer seals essential?

Threads and bolt heads are a common source of leaks in hydraulic systems, pipework, and machinery. A washer seal stops these leaks with a simple, low-cost part that fits straight under the bolt, with no need for sealant or extra machining. Because the metal ring controls how far the rubber is squeezed, the seal is consistent and can cope with high pressure.

Washer seals are quick to fit and replace during maintenance, and they are made in standard thread sizes, so the right size is easy to find.

Typical applications include:

• Hydraulic fittings, adaptors, and banjo bolts
• Drain plugs and filler plugs
• Pipe joints and pipework connections
• Valve bodies and pump housings
• Gearboxes and engines
• Pneumatic and compressed air systems
• Fuel and oil lines
• Agricultural and construction machinery

The main types of washer seals

• Standard bonded sealing washers - A metal washer with a bonded rubber ring, for general sealing under bolt heads and fittings.
• Self-centring bonded sealing washers - These have a thin rubber flap on the inside of the bore, which centres the washer on the bolt or thread. This makes fitting easier and makes sure the rubber seals evenly all the way round.
• Nitrile washer seals - The standard choice, with nitrile rubber that resists mineral oils, hydraulic fluids, greases, and fuels.
• FKM washer seals - Washer seals with an FKM rubber ring, for higher temperatures and more aggressive fluids.
• Metric and BSP sizes - Washer seals made to fit metric bolts and threads, such as M10 to M24, and British Standard Pipe threads, such as 1/8 inch to 1/2 inch BSP.

How to choose the right washer seals?

• Thread size - Choose a washer seal that matches the bolt or fitting thread, whether metric or BSP.
• Standard or self-centring - Choose a self-centring washer seal where it is hard to hold the washer in position while fitting, or where an even seal is especially important.
• Rubber material - Choose nitrile for oils, fuels, and hydraulic fluids, and FKM for higher temperatures or aggressive chemicals. Check the material suits the fluid being sealed.
• Pressure - Washer seals are designed for high pressure, but check the size and material suit the system.
• Corrosion - In wet or corrosive conditions, check whether a plated or stainless steel washer is needed.

Essential references and specialised ranges

How the numbering works:

Washer seals are referenced by type, material, and the thread size they fit:

Self-centring bonded sealing washer M12 = self-centring washer seal to fit an M12 bolt

Bonded sealing washer 3/8 inch BSP = washer seal to fit a 3/8 inch BSP fitting

FKM bonded sealing washer M16 = washer seal with an FKM rubber ring to fit an M16 bolt

Each washer seal has a set inside diameter, outside diameter, and thickness to suit its thread size. For example, a self-centring M12 washer seal is around 19mm across and 1.5mm thick.

Suffixes:

M = metric thread size, for example M12

BSP = British Standard Pipe thread size

NBR = nitrile rubber

FKM = fluoroelastomer rubber

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about washer seals

What is a Dowty washer?

Dowty washer is a common name for a bonded sealing washer, after one of the first companies to make them. It describes the same type of metal washer with a bonded rubber sealing ring.

What does self-centring mean?

A self-centring washer seal has a thin rubber flap inside the bore. This grips the bolt thread and holds the washer in the centre, so the rubber seals evenly all the way round and the washer does not slip out of place while being tightened.

Can washer seals be reused?

It is best to fit new washer seals each time a joint is opened. Once the rubber has been compressed, it may not seal as well a second time.

How tight should a washer seal be?

Tighten the bolt or fitting until it meets the metal washer, following the torque recommended for the fitting. The metal ring stops the rubber being over-compressed, so there is no need to overtighten.

How do I fit a washer seal?

Clean the sealing faces on both the fitting and the housing, and check for burrs or damage. Fit the washer seal over the bolt or fitting, centring it on the thread, and tighten to the recommended torque.

How do I identify the size I need?

Check the thread size of the bolt or fitting, such as M12 or 1/2 inch BSP, and choose a washer seal to suit. If you have the old washer, measure its inside diameter, outside diameter, and thickness.

How can I tell if a washer seal needs replacing?

Warning signs include:

• Fluid weeping or leaking around the bolt head or fitting
• A rubber ring that is cracked, hardened, or squeezed out
• Rust or corrosion on the metal washer
• A washer that has been reused or damaged during removal