Washers · Trade series (vulcanised fibre washer)

A4 (316) Stainless Steel Insulating Fibre Washers

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 Stainless Steel (ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80) is specified for marine, coastal and chemical applications. When insulating fibre washers are specified in this grade, the dimensions remain per Trade series (vulcanised fibre washer); the material determines corrosion behavior (superior, including marine and chloride environments) and the service temperature range (approx. -200 °C to +400 °C, subject to the property class and the joint). Both are covered below with the size table.

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)	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

Technical Drawing

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Specification

Item / category Standard Size(s) Material / finish Quantity

Business Information

Company * Contact Name * Email * Phone (with country code) Country / Region

Notes

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

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

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.

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.
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.
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.
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.