Bonded Seal Size Chart

Choose the Correct Bonded Seal for Reliable Hydraulic Sealing

A bonded seal, sometimes referred to as a Dowty Seal, is a sealing washer designed to prevent fluid leakage around threaded fasteners, plugs, and hydraulic fittings. It combines a precision-machined metal washer with a bonded elastomer sealing ring, providing both mechanical support and effective sealing under pressure.

Selecting the correct bonded seal size is essential for maintaining system integrity. An undersized seal may be difficult to install, while an oversized seal can lead to insufficient compression and premature leakage.

How a Bonded Seal Works

Unlike a conventional flat washer, a bonded seal creates a positive seal by compressing the elastomer element between the fastener head and the mating surface.

When the fitting is tightened:

• The metal washer limits over-compression.
• The elastomer deforms to fill surface irregularities.
• A tight seal is created around the bolt or fitting.
• Fluid leakage is minimized even under high hydraulic pressure.

This combination of strength and sealing performance makes bonded seals suitable for hydraulic and pneumatic systems where reliable pressure retention is required.

Standard Bonded Seal Size Chart

Bonded seals are manufactured in a variety of standard sizes to match common bolt and fitting dimensions.

Typical sizing systems include:

When selecting a bonded seal, confirm the bolt diameter, thread specification, and seating dimensions to ensure proper fit.

How to Select the Correct Bonded Seal Size

Choosing the right bonded seal involves more than matching the thread size.

Consider the following factors:

Bolt or Thread Size

The seal’s inside diameter should fit the fastener without excessive clearance while allowing easy installation.

Sealing Face Diameter

The outside diameter should provide enough sealing surface without interfering with adjacent components.

Operating Pressure

Higher-pressure systems require adequate sealing compression and compatible elastomer materials.

Surface Finish

Bonded seals perform best on clean, smooth sealing faces. Deep scratches or machining defects may reduce sealing effectiveness.

Elastomer Material Options

The rubber insert determines chemical compatibility and operating temperature.

NBR (Nitrile)

Recommended for:

• Hydraulic oil
• Mineral oil
• Diesel fuel
• General industrial applications

Advantages

• Good oil resistance
• Economical
• Suitable for most hydraulic systems

FKM (Viton®)

Recommended for:

• High-temperature hydraulic systems
• Synthetic lubricants
• Fuel systems
• Chemical processing equipment

Advantages

• Excellent heat resistance
• Superior chemical compatibility
• Long service life

EPDM

Recommended for:

• Water-based hydraulic fluids
• Coolant systems
• Steam applications

Advantages

• Excellent water resistance
• Good weathering performance
• Resistant to many cleaning chemicals

Material selection should always be based on the operating fluid, temperature range, and environmental conditions.

Installation Tips

Correct installation has a direct impact on sealing reliability.

For best results:

• Verify that the sealing faces are clean and free of damage.
• Match the bonded seal to the correct thread standard.
• Avoid excessive tightening that may deform the metal washer.
• Replace bonded seals after removal in critical applications to maintain sealing integrity.
• Inspect the elastomer for cuts, aging, or permanent compression before installation.

Following these practices can reduce leakage and extend the service life of hydraulic connections.

Why Proper Sizing Matters

Using an incorrect bonded seal size can lead to:

• Hydraulic oil leakage
• Loss of system pressure
• Increased maintenance costs
• Equipment downtime
• Premature seal wear

A properly selected bonded seal provides reliable sealing performance while protecting hydraulic systems from pressure loss and fluid contamination.

Bonded Seals for Industrial Fluid Systems

Bonded seals offer a practical sealing solution for threaded hydraulic and pneumatic connections by combining the strength of a metal washer with the flexibility of an elastomer seal.

Whether your application uses metric, BSP, or SAE fittings, selecting the correct bonded seal size and material helps improve sealing reliability, reduce maintenance requirements, and support long-term equipment performance.