Bonded Seals (Dowty)
Metal-reinforced hydraulic port sealing — hundreds of bar, no extrusion, no re-torquing after assembly.
Overview
The hydraulic port seal that does two jobs at once
Bonded seals — also known as Dowty seals, after their inventor — combine a metal washer with an elastomer ring bonded to its inner bore face. The metal outer ring carries all the clamping load from the fastener; the elastomer inner ring seals micro-surface imperfections on the port face that no metal washer alone can bridge. The result is a seal that handles several hundred bar of hydraulic or pneumatic pressure with face torques far lower than a compression fitting, and with zero risk of over-crushing the elastomer during assembly.
Originally developed in the 1940s for hydraulic aircraft landing gear, bonded seals are now the standard sealing solution for straight-thread hydraulic ports (BSP, metric, JIC) in mobile equipment, machine tools, and oil & gas manifolds. They seat against a flat port face — the thread itself provides no sealing action, only clamping. This is what distinguishes a straight-thread port system from a tapered NPT system.
We carry bonded seals in standard BSP, metric, and NPT face sizes, in nitrile and FKM elastomers, with carbon steel or stainless 316 metal shells.
Why Bonded Seals
A hybrid design that combines the best of both worlds
The strength of metal and the flexibility of rubber in one small component. Here is how the bonded seal stacks up against the alternatives you are likely already using.
vs Metal washers
100% metal washers — steel, copper, aluminum — are perfect for distributing loads, but they do not provide a seal. Under pressure, the fluid always finds a way through the micro-asperities of the mating surfaces. No matter how tight you torque it, the metal conforms only partially to surface irregularities, and over time vibration and thermal cycling open leak paths that a metal washer cannot close.
The bonded seal solves this problem: its elastomer insert fills these imperfections and creates a seal-proof barrier from the first tightening. The metal part acts as a stop, preventing overcompression of the rubber and ensuring stable contact pressure over time. You get the load distribution of a washer and the sealing of an elastomer — from a single component that installs in seconds.
vs Aramid and composite flat gaskets
A conventional flat gasket can do the job, but often at the cost of high torque and the risk of creep or deformation over time. Aramid and composite gaskets need to be compressed hard enough to flow into surface imperfections — that requires significant bolt torque, and the material continues to creep under sustained load, which means the joint loses clamp force and eventually needs retorquing.
The bonded seal offers the same sealing performance with a much lower tightening torque requirement. It adapts to the bearing surface through elastomer compliance rather than material crushing, remains stable through temperature variations, and withstands numerous disassembly cycles without loss of efficiency. The result: less maintenance, fewer unplanned leaks, and greater long-term reliability on threaded port connections.
vs Standard O-rings
100% rubber seals are flexible, but at high pressure they quickly show their limits: crushing, extrusion into the thread clearance, or cutting on sharp port edges. An O-ring sitting loose against a port face has no mechanical stop — the clamping force goes entirely into the elastomer, and over-torquing destroys the seal or distorts the port face.
The bonded seal eliminates these problems through its metal retaining washer. It keeps the elastomer in place even under pressures of several hundred bar — the metal ring takes the clamp load and the elastomer only does sealing work. This maintains the elasticity needed for a reliable seal while benefiting from the mechanical strength of the metal carrier. For straight-thread hydraulic ports, the bonded seal is simply the better-engineered solution.
Design Reference
Port compatibility & installation guidance
Thread and port standards — know your system
Bonded seals are sized by thread designation, not by a separate seal dimension. A bonded seal for a BSPP G1/2 port will have the correct inner diameter to seat on the G1/2 port face. Verify the thread type before ordering — BSPP and metric threads are not interchangeable even when thread pitch appears similar.
| Thread Standard | Profile | Sealing Method | Bonded Seal Application |
|---|---|---|---|
| BSPP (BSP Parallel / G) | 55° Whitworth, parallel | Bonded seal on port face (ISO 228) | Primary use case. Seal seats on a flat machined face. Standard in European and Commonwealth equipment. |
| Metric Parallel (ISO 6149 / DIN 3852) | 60° metric, parallel | Bonded seal on port face or O-ring face seal (ORFS) | Increasingly common in modern European hydraulics. Size designation: M10×1, M12×1.5, M14×1.5, etc. |
| NPT (National Pipe Taper) | 60°, tapered | Tapered thread seals on thread flanks — not a face seal | Bonded seals are NOT compatible with standard NPT ports. NPT seals on the thread taper itself (with PTFE tape or anaerobic sealant). |
| NPTF (Dryseal) | 60°, tapered | Thread interference seals without sealant | Not a bonded seal application. Requires precision NPTF threading and gauging. |
When converting NPT ports to a bonded seal system, a port insert adapter (straight-thread boss with flat face) is required. Contact us for adapter sizing.
Assembly torque — typical values by thread size
The values below are representative for standard steel fittings with a bonded seal against a clean, flat, undamaged port face. Always verify against the fitting or equipment manufacturer's specification — some manifold designs specify tighter tolerances or surface-treated ports with different friction coefficients.
| Thread (BSPP/ISO) | Metric Equivalent | Typical Torque (N·m) | Notes |
|---|---|---|---|
| G 1/8" | M10 × 1 | 15–25 | Small ports in instrument manifolds. Hand-tight + ¼ turn is a reasonable field reference if torque wrench unavailable. |
| G 1/4" | M14 × 1.5 | 30–45 | Most common size in mobile hydraulics. Finger tight + wrench ¼ to ½ turn. |
| G 3/8" | M18 × 1.5 | 55–75 | — |
| G 1/2" | M22 × 1.5 | 90–120 | Common on pump and motor ports. High torque — verify that manifold material (aluminium vs steel) supports spec torque. |
| G 3/4" | M27 × 2 | 140–180 | — |
| G 1" | M33 × 2 | 190–240 | Large bore return and pressure ports. Use torque wrench — over-torque risk is highest on large aluminium manifolds. |
Values based on ISO 6149 / Parker Hannifin torque reference for straight-thread O-ring face ports with bonded seals. Lubricated threads reduce torque by ~20% — apply same lubrication consistently for repeatable results.
Pressure capability & re-use limits
The pressure limit of a bonded seal assembly is governed by the port face surface condition and the elastomer compound — not the seal design itself. A properly assembled bonded seal in a clean, flat BSP port routinely handles 400–500 bar in hydraulic service. Surface damage and repeat re-use are the limiting factors in practice.
| Parameter | Guideline | Rationale |
|---|---|---|
| Maximum working pressure | Up to 400 bar (5,800 psi) in well-maintained BSP/metric ports | Governed by port face condition and fitting strength — the elastomer seal itself is not the pressure limit. Damaged or corroded port faces reduce this significantly. |
| Re-use cycles | Maximum 2–3 reassembly cycles; replace at each major service | The elastomer ring takes a compression set over time. After repeated cycles, it no longer conforms reliably to port face micro-imperfections. Bonded seals are low-cost — replace on principle. |
| Port face condition | Flat, smooth, free of radial scratches and corrosion. Ra ≤ 1.6 µm | Radial scratches (from improper tooling or thread cutting chips) create a leak path the elastomer cannot bridge. Inspect port face before installing any new seal. |
| Temperature limits | NBR: −40 °C to +120 °C. FKM: −20 °C to +200 °C | High-temperature hydraulic systems (phosphate esters, fire-resistant fluids) require FKM elastomer. Standard NBR degrades rapidly above 120 °C continuous service. |
Material Quick Reference
Elastomer & metal shell options
Two independent choices: the elastomer compound sets fluid and temperature compatibility; the metal shell sets corrosion resistance and mechanical strength. Specify both for your application.
Elastomers
NBR — Nitrile
−40 °C to +120 °C
Petroleum hydraulic fluids, mineral oils, diesel fuel, water-glycol antifreeze. Standard choice for mobile hydraulics, machine tools, and general industrial systems.
FKM — Fluorocarbon (Viton)
−20 °C to +200 °C
Fire-resistant fluids (Skydrol, HyJet, phosphate esters), aggressive chemicals, high-temperature hydraulic systems. Required where NBR reaches its temperature or chemical limit.
Metal Shells
Carbon Steel
Standard grade
Default shell material for most hydraulic and industrial applications. Low cost, adequate strength. Susceptible to corrosion in wet or outdoor environments — specify a protected or alloy shell where moisture is a factor.
Zinc Plated Carbon Steel
Carbon steel + zinc electroplate
Improved corrosion resistance over bare carbon steel at minimal cost premium. Good choice for equipment exposed to condensation, washdown, or intermittent moisture. Not a substitute for stainless in continuously wet or chemical service.
Stainless 304
18/8 austenitic stainless
Good general corrosion resistance for food and beverage, mild chemical service, and indoor industrial equipment. Less resistant than 316 to chloride-bearing environments — do not specify for marine or coastal outdoor service.
Stainless 316
18/10/2 — molybdenum bearing
Superior corrosion resistance including chlorides and salt spray. Standard for offshore, marine, chemical processing, pharmaceutical, and food & beverage washdown applications. Specify where 304 corrodes in service.
Aluminum Bronze
CuAl alloy
Exceptional seawater and marine corrosion resistance. Non-sparking — required for hazardous area and explosive atmosphere equipment. Subsea, offshore, and naval applications where stainless is insufficient or non-sparking compliance is mandatory.
Need bonded seals for your application?
Specify thread standard, size, elastomer, and shell material. Standard and special combinations available — send us the details and we'll source the right seal for your application.
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