Bonded Seals 101: When, Why, and How to Use Them

What is a bonded seal?

A bonded seal — also known as a Dowty seal, sealing washer, or rubber-bonded washer — is a precision-engineered sealing component consisting of a metal washer (typically zinc-plated steel or stainless) with a vulcanized elastomer ring bonded to its inner edge. When compressed under a fitting head or banjo bolt, the elastomer flows into the gap between the bolt seat and the mating surface, creating a leak-tight metal-to-metal seal at relatively low torque.

Invented in the 1940s by the Dowty Group (UK) for aerospace hydraulics, bonded seals have become the industry standard for sealing threaded hydraulic fittings, hose connections, and high-pressure plumbing — replacing the older copper, aluminum, and fiber sealing washers that required higher torque and frequent replacement.

How a bonded seal works

The mechanism is elegant: the metal washer carries the bolt-clamping load and prevents over-compression of the elastomer, while the bonded rubber ring is precisely sized to fill the void between the bolt under-head and the mating surface. As torque is applied:

1. The metal washer bottoms out against both surfaces, transferring the clamping force
2. The elastomer ring is compressed by a controlled amount (typically 15–25%)
3. The compressed elastomer flows radially, filling all surface imperfections
4. A leak-tight seal is established at a fraction of the torque required by a flat metal washer

Because the metal washer prevents over-compression, the seal is reusable for several disassembly cycles (unlike copper crush washers, which must be replaced every time).

When to use a bonded seal

Bonded seals are the correct choice when you need to seal a threaded fitting against fluid pressure, especially in these scenarios:

• BSP, BSPP, and BSPT hydraulic ports
• Banjo bolt assemblies on brake lines, fuel lines, and hydraulic distribution blocks
• Pressure-port plugs on pumps, valves, and accumulators
• Instrument tap-off points on pressure gauges and transmitters
• Coolant and air drain plugs on engines and compressors
• Any application where thread sealant (PTFE tape, anaerobic sealant) is undesirable due to contamination risk

When NOT to use a bonded seal

• Tapered threads (NPT, BSPT) where the thread itself provides the primary seal
• O-ring face-seal (ORFS) fittings, which use a dedicated O-ring instead
• Static-flange applications where a full-face gasket is appropriate
• Cryogenic temperatures below the elastomer's glass transition
• Extreme high-temperature environments above the elastomer's continuous service rating

Standard sizes and the BSP convention

Bonded seals are most commonly specified by the BSP (British Standard Pipe) thread size of the matching fitting. The standard range covers most industrial hydraulic and pneumatic applications:

BSP Thread	Inner Dia. (mm)	Outer Dia. (mm)	Thickness (mm)
1/8" BSP	9.7	14.0	1.5
1/4" BSP	13.0	18.0	1.5
3/8" BSP	16.7	23.0	1.5
1/2" BSP	21.0	27.7	1.5
5/8" BSP	23.0	30.0	2.0
3/4" BSP	26.7	32.0	2.0
1" BSP	33.3	40.0	2.0
1-1/4" BSP	42.0	50.0	2.5
1-1/2" BSP	48.0	56.0	2.5
2" BSP	60.0	69.0	3.0

Metric (DIN), JIS, and SAE bonded seal standards also exist; always verify the OEM's specification before substituting between standards.

Material options

Standard combination: Zinc-plated steel washer + NBR ring

The default choice for 80% of industrial hydraulic applications. Suitable for mineral-oil-based hydraulic fluids up to 100 °C. Cost-effective and widely stocked.

Zinc-plated steel washer + FKM (Viton) ring

Used when operating temperatures exceed 100 °C, when synthetic hydraulic fluids are present, or when extended service life is required. Common in aerospace ground-support, oil & gas, and high-temperature industrial hydraulics.

Stainless steel washer + EPDM ring

Specified for water-based hydraulic systems (HFC fluids), water-glycol fire-resistant fluids, and corrosive environments (marine, chemical processing). Avoid with mineral oil — EPDM is incompatible.

Stainless steel washer + FKM ring

Premium combination for harsh environments where both corrosion and elevated temperature are present — chemical processing, food and beverage with aggressive cleaning chemistry.

Stainless steel washer + PTFE-faced elastomer

For chemical-handling applications where the seal contacts aggressive chemicals (acids, ketones, certain solvents). The PTFE face provides the chemical resistance while the underlying elastomer provides the resilience.

Installation best practices

1. Inspect the seating surface. Both the bolt under-head and the mating port must be flat, clean, and free of nicks or burrs.
2. Use the correct torque. Apply the fitting manufacturer's specified torque value — over-torquing risks shearing the elastomer; under-torquing causes leaks.
3. Do not reuse a damaged seal. Even though bonded seals are technically reusable, inspect the elastomer ring for cuts, swelling, or hardening before reinstallation.
4. Don't combine with thread sealant. The bonded seal is the seal — adding PTFE tape or anaerobic sealant can interfere with proper seating.
5. Match the seal to the fitting standard. A BSP bonded seal will not seat correctly on a metric fitting and vice versa.

The four most common failure modes

1. Elastomer extrusion. Caused by over-torquing or pressure spikes exceeding rated capacity. Switch to a higher-durometer compound or upgrade to a metal-backed seal.
2. Chemical swelling. Seal becomes soft and deformed. Indicates wrong elastomer for the fluid — verify chemical compatibility chart.
3. Hardening and shrinkage. Caused by temperature excursion above rated range. Upgrade compound (NBR → HNBR → FKM as needed).
4. Galvanic corrosion of the metal washer. Mismatched washer and fitting metals (zinc-plated steel against bronze, for example) in wet environments. Switch to a stainless washer.

Frequently Asked Questions

Are bonded seals the same as Dowty seals?

"Dowty seal" is the original brand name (after the Dowty Group, the inventor); "bonded seal" is the generic term. Today both terms are used interchangeably in the industry.

Can a bonded seal replace a copper crush washer?

In most low- to medium-pressure applications, yes — and with significant advantages: lower torque required, reusability, and better long-term sealing. Always verify pressure rating compatibility first.

What's the maximum pressure rating for a standard bonded seal?

Standard NBR bonded seals are typically rated to 250 bar (3,600 psi); high-pressure designs and FKM versions can handle 400–600 bar (5,800–8,700 psi). Always confirm with the supplier datasheet for your specific size and material.

Are bonded seals available in stainless steel?

Yes. Stainless 304 and 316 washer options are available for corrosion-prone environments. Combine with FKM or EPDM rings depending on the fluid and chemistry.

Where can I buy bonded seals in the United States?

Tereefek Solutions stocks the complete ERIKS bonded seal range — BSP and metric sizes, zinc-plated and stainless washer options, NBR / FKM / EPDM elastomer rings — for same-day shipping. Browse our bonded seal catalog or request a quote with your fitting size and pressure requirements.

Bottom line for buyers

Bonded seals are the modern default for sealing threaded hydraulic and pneumatic fittings, replacing older crush-washer technology with lower torque, better reusability, and longer service life. For 80% of applications, a standard zinc-plated steel washer with an NBR ring will perform flawlessly. Upgrade to FKM for high temperatures, EPDM for water-based fluids, and stainless steel washers for corrosive environments.