Dowty Washer Guide: Bonded Seals for Hydraulic & BSP Fittings

A Dowty washer — also called a bonded seal washer — is a metal washer with a vulcanised rubber ring bonded permanently to its inner bore. When the fitting is tightened, the rubber compresses against a flat machined face and forms a leak-tight static face seal. The metal washer acts as a hard stop, limiting how far the rubber is squashed and giving a controlled, repeatable seal. Standard on BSP parallel ports, hydraulic adapters, fuel and lubrication unions, gauge ports and instrumentation. They only work on parallel-thread fittings — never on tapered thread.

Nominal sizes — outer diameter varies slightly between manufacturers. Always check the fitting drawing if there's a tight spotface.

What Is a Dowty Washer

"Dowty washer" started life as a brand name. Dowty Seals Ltd, a British engineering firm founded by Sir George Dowty, patented the bonded seal washer design in the 1940s and became the dominant supplier through the post-war hydraulic boom — particularly for British military aviation hydraulics running at 3,000 psi. The name stuck. Today the company is part of GKN Aerospace, the patent has long expired, and dozens of manufacturers — Hutchinson, Trelleborg, Garlock, James Walker and a long tail of generic suppliers — produce the same design. The generic engineering term is bonded seal washer (sometimes self-centring washer or self-sealing washer), but tradies and parts catalogues across Australia still call them Dowty washers.

The design solves a specific problem: how to seal a bolted joint or threaded fitting reliably without thread tape, anaerobic sealant or a separate O-ring groove. The bonded seal does it in one part — a stamped metal washer with rubber moulded and vulcanised directly to its inner edge. Drop it under the bolt head or fitting shoulder, tighten to spec, and the rubber compresses to form a face seal against the mating surface. No mess, no cure time, no thread prep.

How a Bonded Seal Works — the Controlled-Compression Principle

The mechanics are straightforward. The metal washer carries the bolt clamping load — the same as a flat washer would. The rubber ring bonded to the inner bore sits proud of the metal washer's face by a controlled amount (typically 0.3–0.6 mm) when uncompressed. When the bolt or fitting is tightened, three things happen in sequence:

1. The rubber contacts the mating face first and starts to compress before the metal washer is fully seated.
2. The rubber deforms radially into the gap between the bolt shank and the bolt hole, filling any micro-irregularities in the mating surface.
3. The metal washer bottoms out against the mating face, stopping further rubber compression at the design value. The rubber is now squeezed to roughly 70–80% of its free height — enough to seal, not so much that it splits or extrudes.

That last step is the clever bit. Without the metal washer acting as a hard stop, a torque-controlled assembly process would either under-squash the rubber (leak) or over-squash it (split, extrude, fail in a few months). The bonded seal is self-limiting by geometry, so it tolerates a wide range of installation torques without losing its seal — exactly what you want on a workshop floor where fitters use rattle guns and feel rather than calibrated torque wrenches.

One consequence: the bond between rubber and metal is the most critical part of the washer. A cheaply made bonded seal where the rubber peels away from the metal under fluid pressure will fail in service even though the install torque was correct. This is why engineering-grade brands cost more than $0.20 generic eBay parts — the surface preparation, primer, vulcanisation cycle and quality control on the bond.

The #1 Mistake — Parallel vs Tapered Thread

This is worth unpacking because it's so common. Australian industry runs almost exclusively on BSP, but there are two variants that look identical to the untrained eye and seal in completely different ways:

• BSPP (BSP Parallel, sometimes called BSP-G or just G thread) — same diameter all the way along the thread. Seals against a flat machined face at the bottom of the male thread or under the head — using a bonded seal washer, copper crush washer or O-ring. The thread itself does not seal. You can spin the male into the female by hand all the way home with no resistance.
• BSPT (BSP Tapered, sometimes called BSP-R or just R thread) — gets fatter as you go along the thread. Seals by metal-to-metal wedging of the male and female thread flanks. Needs thread sealant (PTFE tape, Loctite 567, Loctite 577) to fill the spiral leak path between the engaged threads. You feel the male tighten up halfway in — that's the taper engaging.

NPT (American National Pipe Tapered) is similar to BSPT but with a different thread angle (60° vs BSPT's 55°), and the two are not interchangeable despite looking similar.

How to spot the difference in the workshop: Take the fitting and try to thread the male into the female by hand. If it spins all the way home easily and stops at a flat shoulder — BSPP, use a bonded seal. If it tightens up partway in with no visible shoulder to seat against — BSPT or NPT, use thread sealant. If you put a bonded seal under a BSPT fitting, the rubber has nothing flat to compress against and the joint will weep oil within hours of pressurisation. Conversely, if you wrap PTFE tape on a BSPP fitting, the tape stops the bonded seal seating correctly and the joint will leak even though it feels tight.

Forum reality check (Yesterday's Tractors, Practical Machinist threads on persistent BSP leaks): the second most common cause of BSP leaks after thread-type confusion is losing the bonded seal during disassembly. The rubber-bonded washer is small, dark and often stuck to the fitting shoulder by old oil. It can fall off into a drip tray during a service and get binned. Without it, a BSPP fitting cannot seal regardless of how tightly it's torqued — and the next mechanic to look at it spends an hour chasing a leak that's actually a missing $0.50 part. Always check the seal is there and replace it if there's any doubt.

Rubber Materials — NBR, Viton/FKM, EPDM

The metal washer is almost always carbon steel with a zinc-plated or zinc-and-clear-passivate finish. Stainless steel (304 or 316) is available for marine, food contact or aggressive chemical service. The metal selection is straightforward — match the bolt and fitting material to avoid galvanic corrosion in wet service.

The rubber is where most of the selection thinking happens. Three compounds cover roughly 95% of Australian industrial use:

NBR is the default. Unless the application calls for something specific — hot oil, brake fluid, food contact, aggressive chemistry — assume NBR. AIMS stocks NBR bonded seals across the full BSP and metric range because that's what 90% of jobs need.

Forum-validated compatibility traps:

• Ethanol-blend fuels (E10, E85) — automotive forums repeatedly flag NBR bonded seals failing within 12 months on ethanol-blend fuel lines. The ethanol leaches plasticisers out of the nitrile and the rubber hardens, shrinks and cracks. Use FKM (Viton) for any fuel system fitting that will see E10 or higher.
• Brake fluid — never NBR or FKM. DOT 3/4/5.1 fluids are glycol-based and chemically attack both. Use EPDM. DOT 5 (silicone-based) is the exception — silicone fluid is compatible with NBR. Mechanics regularly get caught out on this swap.
• Hot mineral oil >100°C — NBR hardens and cracks at sustained temps above 100°C. If the fitting is on an engine block or hydraulic return line near a heat source, step up to FKM.
• Refrigeration systems (HVAC) — refrigerant oils and HFC refrigerants need FKM. NBR will swell.

Sizing — BSP, Metric, UNF

Bonded seal washers are sized by the bolt or thread they fit, not by an arbitrary part number. The two dimensions that matter are the bore (must be a slip fit over the male thread) and the outer diameter (must fit within the spotface or counterbore on the female component).

Metric (M) and BSPP sizes overlap because most hydraulic and pneumatic component manufacturers use metric bolts with parallel-thread (M-Parallel) and the bonded seal range was sized to fit both standards. UNF sizes (1/4"-28, 5/16"-24, 3/8"-24 etc.) are stocked for older British and US-spec equipment but make up a small fraction of Australian industrial use.

Common Applications

Bonded seal washers turn up everywhere a flat-face seal is needed against a parallel-thread fitting or bolted joint:

• Hydraulic ports — every BSPP port on a pump, valve, cylinder, manifold or hose tail across mobile hydraulics, industrial hydraulics and aerospace.
• Pneumatic fittings — air compressor outlets, BSPP air-line manifolds, regulator inlets, FRL (filter-regulator-lubricator) groups.
• Fuel system unions — diesel return lines, fuel filter housings, injector pump fittings (Viton needed for modern diesel and ethanol-blend petrol).
• Lubrication and grease fittings — banjo bolt feeds on automatic lubricators, central lubrication system manifolds.
• Gauge ports and instrumentation — pressure gauge fittings, transducer bosses, sample ports — small sizes (1/8", 1/4" BSPP).
• Sump plugs and drain plugs — engine oil pans, gearbox housings, hydraulic reservoirs. Many OEM sump plugs ship with a bonded seal or aluminium-bonded equivalent.
• Brake banjo fittings — increasingly used on modern motorcycle and automotive brake banjos as a more forgiving alternative to copper crush washers (must be EPDM rubber for DOT 3/4/5.1 brake fluid — read the next section).
• Compressed gas and refrigerant fittings — refrigeration service ports, gas regulator outlets (FKM rubber, never NBR).

Dowty Washer vs Copper Crush Washer vs O-Ring vs Loctite Pipe Sealant

Four different ways to seal a threaded fitting or bolted port. Picking the wrong one is a guaranteed leak. Quick comparison:

Common mismatches and what goes wrong:

• Bonded seal on tapered thread → no flat face to compress against, rubber distorts and weeps within hours. Use thread sealant.
• PTFE tape on BSPP port → tape sits under the bonded seal and stops it seating against the spotface. Apparent tightness, slow weep. Remove tape, install bonded seal alone.
• Copper crush washer reused → work-hardened from first install, won't deform enough on second torque. Use new every time, or switch to bonded seal.
• NBR bonded seal on brake fitting with DOT 4 fluid → rubber swells and softens, seal fails within months. Use EPDM bonded seal or copper crush.
• O-ring boss fitting (SAE J1926) tightened without the O-ring → no seal at all; flat washer underneath does nothing without the elastomer. Always check for and install the correct O-ring.

Installation — Which Way Does the Rubber Face?

The bonded seal is asymmetric — the rubber sits proud on one face of the metal washer and is flush with the other. The convention across hydraulic component manufacturer documentation (Hutchinson, Trelleborg, James Walker, Parker) and verified across mechanic forums is:

The rubber face contacts the mating face (the flat machined surface being sealed). The metal back faces the bolt head or fitting shoulder.

This is because:

1. The bolt head or fitting flange is hard, machined steel — there's nothing to seal against that. The rubber serves no purpose between bolt head and metal washer.
2. The mating face (the port spotface or component face) is where the leak path is. The rubber needs to be against that face to compress and seal it.
3. The metal back distributes the bolt clamping load evenly across the rubber — it acts as a follower plate.

In practice it's hard to install one upside down because the rubber-proud face is visually obvious. But on small sizes (1/8" BSPP) it's worth a deliberate check before tightening — particularly if you're working blind in a confined space.

Other installation rules:

• Mating face condition — the seal will only seal as well as the surface it compresses against. Wipe the spotface clean of old sealant residue, oil and grit before assembly. A nick or scratch radial across the spotface will give a permanent weep.
• No thread tape, no Loctite, no extra goop — a bonded seal on a parallel thread needs nothing else. Adding PTFE tape under it actively prevents it sealing. Adding Loctite 577 won't hurt the seal but is pointless on a parallel thread.
• Torque — most BSPP fittings have a recommended torque in the component manual. As a rough guide for hydraulic adapters: 1/4" ~ 25 Nm, 3/8" ~ 50 Nm, 1/2" ~ 90 Nm, 3/4" ~ 175 Nm. The bonded seal is forgiving — within ±25% of these figures it will seal. Severe over-torque can split the rubber.
• One direction of rotation — tighten in one continuous motion to spec. Don't tighten, back off and re-tighten — that disturbs the rubber and can leave a witness line on the mating face that becomes the leak path on reassembly.

Single-Use Rule — Why Bonded Seals Are (Mostly) Single-Service

Manufacturer specification sheets often allow limited reuse of bonded seals if undamaged. Mechanic forum consensus (Mini Forum, MG Experience, Practical Machinist, Yesterday's Tractors) is firmly against it for any pressurised system:

• The rubber takes a compression set on first install. When you back the fitting off, the rubber doesn't fully spring back. The next install starts with less rubber height available to seal.
• If the rubber has been heat-cycled (engine bay, hot hydraulics) the compound has aged in place. Refitting introduces a stiffer, less compliant seal to a fresh mating face.
• Any nick or scratch on the rubber from disassembly tools — pick, screwdriver, fingernail — becomes a leak path.
• The cost of a bonded seal is typically $0.30–$2.00. The cost of chasing an intermittent hydraulic leak through a multi-fitting circuit is hours of labour and a customer return.

Rule of thumb: if the fitting comes apart, the bonded seal gets replaced. The only exception is dry, low-pressure pneumatic work where a quick disassembly-reassembly within minutes (e.g. setting fitting orientation on a new install) is reasonable.

Note that this is more conservative than the supplier line. Suppliers state bonded seals can be reused. Real-world maintenance practice on production hydraulics is to use new every time — labour cost of investigating a recurring weep dwarfs the parts cost ten times over.

AIMS Industrial Bonded Seal / Dowty Washer Range

AIMS Industrial stocks bonded seal washers across the full BSP parallel and metric range used in Australian hydraulics, pneumatics and fluid handling. NBR is our standard stock compound — Viton (FKM) available on order for fuel system, hot oil and refrigerant applications.

Browse the range:

• Sealing & Cushioning Washers — full bonded seal range plus crush washers and cushioning washers
• All Washers — flat, spring, structural, bonded seal and specialty
• Hydraulic Fittings — BSPP adapters, hose tails, banjo unions and the bonded seals to suit
• Hydraulic Components — pumps, valves, cylinders, hose
• Air Tools & Pneumatics — pneumatic fittings, regulators and the bonded seals they need
• Loctite Range — for the tapered-thread fittings that need thread sealant instead
• Thread Sealants — Loctite 567, 577 and PTFE tape for BSPT/NPT fittings

Not sure which seal you need or what's leaking? Call our team on (02) 9773 0122 — bring or send a photo of the fitting and the male thread, and we'll match the right seal for the fluid, temperature and pressure. We stock for the trade so we know what's actually used on Australian shop floors, not just what's in the catalogue.

FAQ — Dowty Washers and Bonded Seals

What's the difference between a Dowty washer and a bonded seal?
There is none. Dowty washer is a brand name (Dowty Seals Ltd, UK) that became the generic Australian term for bonded seal washers. The patent expired decades ago and the term now describes the design, not the brand. Most washers stocked as "Dowty washers" in Australian fastener catalogues are made by Hutchinson, Trelleborg, James Walker or generic Asian manufacturers.

Can I use a bonded seal on an NPT or BSPT fitting?
No. Bonded seals only work on parallel-thread fittings with a flat spotface to compress against. NPT and BSPT seal by metal-to-metal wedging of tapered threads — there's no flat face for the rubber to seal against. Use Loctite 567, Loctite 577 or PTFE thread tape on tapered fittings. This is the most common cause of leaks among DIY hydraulic installers.

Which way does the rubber face — towards the bolt or towards the seal face?
The rubber face contacts the flat machined surface being sealed (the port spotface or component face). The metal back of the washer sits against the bolt head, fitting shoulder or flange. This is consistent across all major manufacturer documentation. On smaller sizes it can be hard to tell visually — feel for the slightly proud rubber side and put that towards the sealing face.

Can I reuse a Dowty washer?
Manufacturers say yes if undamaged. Production hydraulic mechanics say no — always use new on any pressurised fitting. The cost of a bonded seal is $0.30–$2.00; the cost of chasing a recurring weep is hours of labour. The rubber takes a compression set on first install and doesn't fully spring back, so the second install starts with less seal height. Reuse is reasonable on dry low-pressure pneumatic work for trial-fitting purposes.

What rubber compound do I need for hydraulic oil?
NBR (Nitrile, Buna-N) is the standard compound for mineral hydraulic oil — covers HLP, HM, HV grades and standard ISO VG 32/46/68. For hot hydraulic systems running >100°C return-line temperature, or for fire-resistant fluids (HFC water-glycol, HFD phosphate ester), step up to FKM (Viton). For water-glycol HFC use EPDM. Never use NBR on phosphate ester (HFD) — it swells and fails.

Can I use a Dowty washer on a brake banjo bolt?
Only if the rubber is EPDM. Standard NBR bonded seals will fail on DOT 3, DOT 4 or DOT 5.1 brake fluid — the glycol-based fluid attacks the nitrile rubber. EPDM bonded seals are compatible. Note that DOT 5 (silicone-based) brake fluid is the exception — NBR is fine with DOT 5. Most automotive brake banjos still use copper crush washers as the OEM seal because copper is universally fluid-compatible. If you're switching to a bonded seal on a brake fitting, confirm the rubber compound first.

Why does my BSP fitting still leak with a new Dowty washer?
Five likely causes in rough order of frequency: (1) it's actually a BSPT fitting not BSPP — check by hand-threading the male and seeing if it stops at a flat shoulder or tightens up partway in; (2) PTFE tape was wrapped on the thread under the bonded seal — remove the tape, the seal needs direct contact with the spotface; (3) the spotface is scratched or has old sealant residue — clean it and check for a radial nick; (4) the wrong size washer is being used and isn't compressing properly; (5) the bonded seal is upside down — rubber must face the sealing surface.

Do I need to use thread sealant with a Dowty washer?
No. On a BSPP parallel-thread fitting with a bonded seal, no thread tape, no Loctite and no liquid sealant should be applied. The bonded seal does the sealing on its own at the spotface. Adding PTFE tape actively prevents the bonded seal seating and is the second most common cause of BSPP leaks after thread-type confusion.

What torque should I use on a bonded seal fitting?
Follow the component manufacturer's torque spec where available. As a working guide for hydraulic BSPP adapters: 1/8" ~ 15 Nm, 1/4" ~ 25 Nm, 3/8" ~ 50 Nm, 1/2" ~ 90 Nm, 3/4" ~ 175 Nm, 1" ~ 300 Nm. The bonded seal is forgiving — within ±25% of these values it will seal. Severe over-torque can split the rubber and create a leak.

Are bonded seals OK for fuel systems?
Yes for diesel and traditional petrol with NBR (Nitrile). For modern Australian ethanol-blend petrol (E10, E85), upgrade to FKM (Viton) — NBR hardens and cracks within 12 months on ethanol-blend fuel. For LPG and gas systems, check the rubber compatibility with the specific gas — Viton is usually safe, NBR is hit-and-miss.

What's the difference between a Dowty washer and an O-ring face seal fitting (ORFS)?
A Dowty washer is an add-on component — slip it under any flat-face BSPP fitting. An O-ring face seal fitting (ORFS, SAE J1453) is a fitting type with a machined O-ring groove built into the face. Both seal by elastomer compression against a flat face. ORFS is the higher-reliability standard on premium hydraulics because the O-ring is captive in a groove and can't fall out. Dowty washers are more flexible because they fit any standard BSPP port — no special machining required.

Can I make a bonded seal at home from a flat washer and an O-ring?
For very low-pressure pneumatic or static water applications, yes — combining a flat washer with an O-ring underneath approximates the bonded seal function. For any hydraulic or pressurised fluid application, no. The bonded rubber-to-metal vulcanisation is what stops the rubber extruding sideways under pressure. A loose O-ring on a flat washer will squeeze out radially and the joint will weep at any meaningful pressure.

Why are stainless steel bonded seals more expensive?
Stainless steel (304 or 316) bonded seals run 3–6× the price of zinc-plated carbon steel equivalents because of raw material cost and because bonding rubber reliably to passive stainless surfaces requires more aggressive surface preparation. They're worth it in marine, food contact, pharmaceutical and aggressive chemical service where carbon steel would rust at the bond line and break the seal.

Where do I find the right size bonded seal for my fitting?
Match the bore to the male thread size and check the outer diameter fits inside any counterbore on the female component. AIMS keeps the standard BSPP and metric range in stock — call us on (02) 9773 0122 with the thread size of your fitting and we'll match it. If you're unsure of the thread, send a photo or bring the fitting in to our Milperra warehouse.