A sealing washer is the right call whenever a bolted joint must hold a fluid or gas inside, or keep dust and water outside; the deciding factors are the media, operating pressure and temperature, and the elastomer's chemical compatibility [S3].
Within the broader washer family, sealing washers sit alongside flat, spring, and lock washers, but their job is different: they deform or compress to fill the micro-gap under a fastener head and stop leak paths that flat hardware cannot close [S1].
Defining the duty: where a sealing washer actually earns its slot
A sealing washer is a flat or shallow-formed disc that integrates an elastomer (EPDM, NBR, FKM, silicone, neoprene) or a soft metal gasket face to close a fluid or gas path under a bolt head [S2]. Standard flat washers spread load; sealing washers add a leak-tight interface without a separate gasket groove [S1].
Three duty classes dominate. Low-pressure enclosure seals (junction boxes, outdoor fixtures, roofing screws) typically run under 1 bar and rely on rubber-bonded metal washers. Mid-pressure plumbing and pneumatic lines run 1–10 bar and use EPDM or NBR bonded washers. High-pressure hydraulic and fuel joints above 10 bar move to crush washers, O-ring boss washers, or bonded steel-rubber combinations [S3][S5].
Material matrix: elastomer, metal carrier, and what each combination tolerates
Material choice drives both chemical resistance and temperature window; the wrong elastomer swells, hardens, or cracks within weeks [S2][S4].
NBR (nitrile) handles petroleum oils, greases, and hydraulic fluid up to roughly 100–110°C continuous and remains the default for general hydraulic and diesel work [S3]. EPDM resists water, steam, glycol, and many acids up to about 150°C and is the standard pick for potable water, plumbing, and outdoor roofing screws [S4]. FKM (Viton-class) covers aggressive fuels, solvents, and hot oil up to around 200°C and is specified for engine and chemical-plant joints [S2].
Neoprene sits between NBR and EPDM for general weathering and ozone, used in HVAC and marine fittings. Silicone covers food-grade and high-temperature dry-heat service. The metal carrier underneath is usually stainless steel (A2 or A4 grade) for corrosion duty, carbon or galvanized steel for cost-driven structural work, and aluminum only for light-duty decorative assemblies [S1][S2].
Sizing: inside diameter, outside diameter, and thickness
Selection starts with three numbers: internal diameter (ID) must clear the bolt shank without binding, outside diameter (OD) must cover the mating flange or hole counterbore, and thickness drives the compression ratio under torque [S4].
A practical rule is to size ID at bolt nominal + 0.2–0.5 mm clearance for through-holes, while OD is typically 1.5–2.0× the bolt diameter for general sealing and 2.5×+ for soft materials like thin sheet or GRP [S4]. Thickness for rubber-bonded sealing washers commonly runs 1.0–3.0 mm depending on durometer, with 50–70 Shore A the typical working range; lower durometer (40–50A) seals rougher surfaces, higher durometer (70–80A) carries higher pressure with less extrusion [S2].
Type comparison: bonded, crush, O-ring boss, and fibre/rubber-faced
Four sealing-washer styles cover the bulk of industrial use, and the right one depends on pressure, reusability, and assembly torque control [S3][S5].
Rubber-bonded sealing washers (stainless or galvanized steel carrier with molded EPDM or NBR face) suit plumbing, roofing, and outdoor enclosures up to roughly 10 bar and are usually single-use. Crush washers, typically soft copper, aluminum, or nylon, deform plastically on first torque-up to fill micro-gaps and are common on drain plugs, banjo bolts, and low-to-mid pressure hydraulic fittings; they are reusable only when the surface remains smooth and undistorted [S5]. O-ring boss or O-ring sealing washers carry a captive elastomer ring for high-pressure hydraulic and pneumatic ports above 10 bar, including instrumentation and process lines tied to pressure transmitter manifolds. Fibre, rubber, or PTFE-faced washers give a flat, low-profile seal where bolt head geometry limits thickness.
Who should NOT default to a standard rubber-bonded sealing washer
A bonded EPDM or NBR sealing washer is the wrong pick on three duty profiles: high-temperature exhaust and steam headers above 150°C, aggressive chemical service requiring FKM or PTFE, and any joint that must be opened and resealed repeatedly with no torque control [S2][S3].
For these cases, move to FKM- or PTFE-faced sealing elements, metal-jacketed gaskets, or O-ring boss assemblies with controlled compression. Specifying a bonded washer in a 200°C hydrocarbon line, or relying on a crush washer as a multi-service seal, leads to extrusion, loss of preload, and leak path reopening within a few thermal cycles [S2][S5].
Use cases and standards alignment
Sealing washers appear in plumbing fittings, HVAC flanges, hydraulic valve ports, instrumentation manifolds, and roofing or cladding fasteners. Specifying them alongside flow meter and pressure transmitter process connections is standard practice where the impulse line or manifold must hold leak-tight under thermal cycling [S3].
Materials are commonly referenced against ASTM D2000 line call-outs for elastomer grade, ISO 3601 for O-ring geometry, and ASTM F436 / F844 for the underlying flat-washer geometry when a sealing face is bonded to a standard washer blank. Drinking-water service in Europe and the UK typically requires WRAS or KIWA approval on EPDM components; potable-water and food-grade washers must document compliance rather than rely on generic datasheet claims [S4].
Limits, failure modes, and sourcing signals
The dominant failure modes are elastomer extrusion under high clamping force, chemical swelling that drops hardness, and compression set that prevents re-seal after thermal cycling; each is driven by the wrong material, wrong durometer, or wrong thickness for the working gap [S2][S3].
Track these signals when sourcing: confirm the elastomer family and Shore A value on the data sheet (not just "rubber"), request a durometer and tensile value, and verify the metal carrier grade when the joint sees corrosion duty [S1][S2]. For trade buyers, two trackable nodes into Q3–Q4 2026 are the rise of FKM-bonded sealing washers for chemical and hydrogen service and tighter WRAS/KIWA documentation on EPDM plumbing lines. Buyers refining process-side sealing architecture can cross-check the broader sealing spec map and review bellows seal selection for higher-class valve joints where a washer alone will not suffice.