--- title: "A4 (316) Stainless Steel Insulating Fibre Washers" description: "A4 (316) Stainless Steel Insulating Fibre Washers (ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80): superior, including marine and chloride" url: https://engineeringhardware.com/guide/material/a4-stainless-steel-insulating-fibre-washers/ source: Engineering Hardware & Fasteners --- # A4 (316) Stainless Steel Insulating Fibre Washers Washers · Trade series (vulcanised fibre washer) Mechanical properties, corrosion behaviour and the full dimensional range for this grade. A4 Stainless Steel Insulating Fibre Washers are supplied to ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80. Corrosion behaviour is superior, including marine and chloride environments, the material is largely non-magnetic, and the usable service range is approx. -200 °C to +400 °C, subject to the property class and the joint. In practice this grade is specified for marine, coastal and chemical applications. - **Grade:** ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80 - **Corrosion resistance:** Superior, including marine and chloride environments - **Magnetic response:** Largely non-magnetic - **Service temperature:** approx. -200 °C to +400 °C, subject to the property class and the joint Property values are typical for the grade. Certified mechanical test data is available on request for production orders. ![A4 (316) Stainless Steel Insulating Fibre Washers reference image](https://engineeringhardware.com/wp-content/uploads/2026/08/Insulating-Washers.webp) A4 Stainless Steel Insulating Fibre Washers are supplied to our own published dimensions — this is a trade-series article rather than one covered by a dimensional standard, so confirm the figures against the part you are replacing. The grade provides superior, including marine and chloride environments and is largely non-magnetic; the practical service range is approx. -200 °C to +400 °C, subject to the property class and the joint. ## Why Choose A4 Stainless Steel for Insulating Fibre Washers Specified for marine, coastal and chemical applications. For insulating fibre washers, this matters when superior, including marine and chloride environments is the primary requirement. ## Mechanical & Physical Properties | Grade designation | ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80 | | --- | --- | | Magnetic response | Largely non-magnetic | | Service temperature | approx. -200 °C to +400 °C, subject to the property class and the joint | | Corrosion characteristic | Superior, including marine and chloride environments | ## Galvanic & Galling Considerations For stainless insulating fibre washers the compatibility question is galvanic rather than galling: joined to aluminum or galvanized steel in wet service, stainless drives corrosion of the less-noble metal. Isolate with a coating, sleeve or non-conductive washer where the joint sees moisture. Note also that stainless is softer than hardened carbon steel, so where the part carries a wear or shear duty, check the load against the grade before substituting. ## Comparable Grades For Insulating Fibre Washers, the alternative stainless grade is A2 (304). Choose A2 (304) for general use and A4 (316) wherever chlorides are present; the molybdenum in A4 is the whole difference. Remember that stainless is the corrosion answer, not the strength answer — for load, look at class 8.8 or 10.9 instead. ## Using Stainless Fasteners in Aluminum Stainless steel is more noble than aluminum, so with an electrolyte present — rain, condensation, salt spray — the aluminum becomes the anode and corrodes at the joint. Area ratio decides how bad it gets: a small stainless fastener in a large aluminum section is normally acceptable, a large stainless fastener in thin aluminum is not. Where the joint gets wet, isolate the metals with a coated or nylon washer and a sealant, or specify an aluminum or coated fastener instead. ## Sizes of Insulating Fibre Washers Available in A4 Stainless Steel | For bolt size | Inner diameter d1 (mm) | Outer diameter d2 (mm) | Thickness t (mm) | Dielectric strength (kV/mm) | Max temperature (u00b0C) | Moisture | | --- | --- | --- | --- | --- | --- | --- | | M3 | 3.2 | 7.0 | 0.5 | 10 | 105 | Hygroscopic u2014 indoor use | | M4 | 4.3 | 9.0 | 0.5 | 10 | 105 | Hygroscopic u2014 indoor use | | M5 | 5.3 | 10.0 | 0.8 | 10 | 105 | Hygroscopic u2014 indoor use | | M6 | 6.4 | 12.0 | 0.8 | 10 | 105 | Hygroscopic u2014 indoor use | | M8 | 8.4 | 16.0 | 1.0 | 10 | 105 | Hygroscopic u2014 indoor use | | M10 | 10.5 | 20.0 | 1.0 | 10 | 105 | Hygroscopic u2014 indoor use | Dimensions are independent of the material: the table above lists the sizes supplied, and on this page every one of them is quoted in A4 Stainless Steel. The catalogue's default material column has been omitted here so it cannot be read as contradicting that. Custom manufacturing · Made to drawing ## Need this washer in a grade that is not listed? A4 Stainless Steel is one of the grades the catalog describes. Bore, outside diameter, thickness, temper and finish are the variables here — there is no thread to specify, so the fit and the bearing area are what the drawing has to pin down. Made by stamping or punching. Minimum order from 2,000 pieces; samples within 7 days of drawing approval. [Custom washer manufacturing](https://engineeringhardware.com/custom/washers/)[Send a drawing](https://engineeringhardware.com/custom/washers/#rfq) ## Technical Drawing Dimensioned line drawing of M3 Insulating Fibre Washers to Trade series (vulcanised fibre washer). Side view and top view, drawn to the dimensions in the table below and redrawn for any size selected. - **Inner diameter d1 (mm):** 3.2 - **Outer diameter d2 (mm):** 7.0 - **Thickness t (mm):** 0.5 - **Dielectric strength (kV/mm):** 10 - **Max temperature (u00b0C):** 105 - **Material:** Vulcanised fibre - **Moisture:** Hygroscopic u2014 indoor use ## Dimensions & Specifications (Trade series (vulcanised fibre washer)) | For bolt size | Inner diameter d1 (mm) | Outer diameter d2 (mm) | Thickness t (mm) | Dielectric strength (kV/mm) | Max temperature (u00b0C) | Material | Moisture | | --- | --- | --- | --- | --- | --- | --- | --- | | M3 | 3.2 | 7.0 | 0.5 | 10 | 105 | Vulcanised fibre | Hygroscopic u2014 indoor use | | M4 | 4.3 | 9.0 | 0.5 | 10 | 105 | Vulcanised fibre | Hygroscopic u2014 indoor use | | M5 | 5.3 | 10.0 | 0.8 | 10 | 105 | Vulcanised fibre | Hygroscopic u2014 indoor use | | M6 | 6.4 | 12.0 | 0.8 | 10 | 105 | Vulcanised fibre | Hygroscopic u2014 indoor use | | M8 | 8.4 | 16.0 | 1.0 | 10 | 105 | Vulcanised fibre | Hygroscopic u2014 indoor use | | M10 | 10.5 | 20.0 | 1.0 | 10 | 105 | Vulcanised fibre | Hygroscopic u2014 indoor use | Product catalog ## 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Applications & Industries The sectors that buy Insulating Fibre Washers, and the duty each one buys them for. Stocked from M3 to M10 (nominal bolt size). ### Industries that specify Insulating Fibre Washers - **Food & pharma equipment:** Non-marking, non-contaminating washers in washdown areas. - **Electrical & electronics:** Insulating and isolating fixings in panels and boards. - **Chemical & process:** Chemically resistant seating under a fastener. - **Machine building & OEM:** Setting bearing area on soft alloy, plastic and oversized holes. - **Timber & construction:** Large-diameter washers that stop pull-through in softer material. - **Rotating & vibrating plant:** Spring, tooth and wedge types that maintain or lock bolt tension. ### Typical applications - protecting a soft or finished surface under a head - insulating a fastener electrically from the panel - separating dissimilar metals to break a galvanic cell - maintaining bolt tension in a joint that settles - spreading clamp load over a soft or oversized bearing face - protecting a painted or plated surface under a turned nut ### Service conditions to specify against - **Marine and coastal:** Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit. - **Hardness matters:** Under a high-strength bolt a soft washer embeds into the joint and preload is lost — specify a hardened washer for structural bolting. [Full application guide: where Insulating Fibre Washers are used, how to specify them, and the cases they are wrong for](https://engineeringhardware.com/applications/insulating-fibre-washers/) Inside the plant ## Manufactured in our own plant This part is cold-formed, threaded, heat-treated and finished in-house. The stages below are where its class and finish are actually decided. 1. ![Wire rod feeding into a cold heading machine](https://engineeringhardware.com/wp-content/uploads/2026/08/09-wire-rod-feed-cold-heading-machine.webp) 01 **Cold heading** Wire rod is drawn to size and cold-formed into blanks. Forming rather than cutting keeps the grain flow continuous around the head, which is where a bolt is most likely to fail. Head dimensions and under-head radius checked at set-up and per shift. 2. ![Thread rolling machine producing bolt threads](https://engineeringhardware.com/wp-content/uploads/2026/08/03-thread-rolling-machine-bolt-production.webp) 02 **Thread rolling** Threads are rolled between hardened dies rather than cut. Rolled threads are stronger in fatigue than cut threads and hold pitch and flank angle far more consistently across a batch. Go / no-go ring gauges to ISO 965 tolerance class 6g. 3. ![Baskets of bolts staged for heat treatment](https://engineeringhardware.com/wp-content/uploads/2026/08/05-heat-treatment-baskets-black-bolts.webp) 03 **Heat treatment** Quench-and-temper brings carbon and alloy steel to the property class ordered — 8.8, 10.9 or 12.9 — with the furnace charge tracked so a finished lot can be traced back to it. Core hardness and tensile tested per lot to ISO 898-1. 4. ![Zinc plated bolts in a treatment basket](https://engineeringhardware.com/wp-content/uploads/2026/08/23-zinc-plated-bolts-treatment-basket.webp) 04 **Surface treatment** Zinc, zinc-nickel, hot-dip galvanizing, black oxide, phosphate and Dacromet, selected against the service environment rather than against the price list. Coating thickness and neutral salt spray hours to ISO 9227. ## Frequently Asked Questions ### What corrosion resistance do A4 Stainless Steel Insulating Fibre Washers offer? A4 Stainless Steel Insulating Fibre Washers provide superior, including marine and chloride environments with a service range of approx. -200 °C to +400 °C, subject to the property class and the joint. ### Should I choose A4 Stainless Steel or A2 (304) for Insulating Fibre Washers? For Insulating Fibre Washers, A4 Stainless Steel suits general indoor and outdoor use; A2 (304) is preferred for marine, coastal and chemical environments. Choose by corrosion exposure first. ### Can A4 Stainless Steel Insulating Fibre Washers gall during installation? Yes — stainless on stainless is prone to thread galling under load, especially at smaller sizes. Use an anti-galling lubricant or wax thread compound, and slow the tightening rate where possible. ### Is A4 Stainless Steel food-grade? A4 (316) is the standard food-contact stainless grade in most jurisdictions, particularly where chloride exposure (brines, dairy CIP) is present. ### How do I request a quotation? Use the technical drawing on this page to pick the exact configuration, then submit it with the quote form. We respond within one business day. ## Related Pages [View the full Insulating Fibre Washers page](https://engineeringhardware.com/fastener/insulating-fibre-washers/) ### More Insulating Fibre Washers Guides