Stainless Steel Plain Washer

Choosing the Right Plain Washer
1) What does a plain washer do & when do you need it?

A plain washer is a flat ring placed under the head of a bolt/screw or under the nut to:

Spread the load over a larger area

Protect the surface (paint, plating, softer materials) from damage

Compensate for slight surface irregularities

Provide a standard bearing surface for consistent tightening

Use a plain washer when:

You are clamping softer materials (aluminium, copper, plastics, wood, sheet metal)

You want to protect paint / powder coating / galvanizing

You are using high-strength bolts and need a proper bearing face

You want more repeatable torque–tension behaviour

2) Environment & corrosion – choosing the washer material

Indoor / dry / general structural & machinery (mild steel parts):

Use MS (mild steel) plain washers, usually zinc-plated or black.

Suitable for general fabrication, machinery, structural steel indoors.

Outdoor / mildly corrosive / galvanized structures:

Use MS plain washers with zinc plating / hot-dip galvanizing.

Match the coating system of bolts and nuts (e.g. HDG bolts → HDG washers).

Stainless structures / food / pharma / decorative:

Use stainless steel washers:

SS 304 (A2) – for general outdoor, food equipment, stainless fabrications

SS 316 (A4) – for coastal, marine, chemical, and wash-down areas

Best practice:

Always match washer material to the bolt & nut material (and environment) as far as possible to avoid galvanic corrosion and staining.

3) Size & standard – OD, ID, and thickness

Plain washers are typically chosen to match the bolt size and standard:

Inner diameter (ID):

Sized to clear the bolt/shank (e.g. M10 washer for M10 bolt).

Avoid very oversized IDs – they reduce the effective bearing area.

Outer diameter (OD):

Standard OD (e.g. DIN 125 / ISO 7089) – for general use.

Large OD / “penny” washers (ISO 7093 type) – for thin sheets, slotted holes, or very soft materials where more load spread is required.

Thickness:

Standard thickness for normal duty.

Thicker washers used for structural joints or where surfaces are irregular and need a more rigid bearing plate.

Rule of thumb:
For regular bolted joints → standard plain washer for that bolt size.
For thin sheet / soft material / slotted holes → choose a larger OD, thicker washer.

4) Type of plain washer – standard vs structural / hardened

For your catalogue you’ll commonly see:

1) Standard plain washer

For everyday fabrication and general mechanical use.

Available in MS (zinc/black/HDG) and stainless (304/316).

2) Hardened / structural washer (for high-strength bolts)

Used with high-tensile bolts (e.g. 8.8 / 10.9) in structural steel joints.

Higher hardness and thickness to resist embedding and brinelling under high preload.

Ensures consistent clamping and prevents loss of tension.

If the joint uses high-strength structural bolts or is subject to heavy cyclic loads, select hardened structural washers instead of soft standard washers.

5) Surface & finish – protecting the joint

MS zinc-plated – for indoor, non-aggressive environments.

MS black (oiled/phosphated) – general mechanical use, often used with black bolts.

Hot-dip galvanized (HDG) – for structural outdoor use, to match HDG bolts and nuts.

Stainless (A2/A4) – for appearance and corrosion resistance.

Make sure the finish matches the bolt/nut finish (especially for structural or outdoor jobs) to maintain overall corrosion performance.

Mechanical Properties (Guide) – Plain Washer

For washers, the key properties are material grade, hardness, and thickness, which together control how well the washer can carry bearing pressure without deforming.

1) Hardened / structural plain washers

These are used with high-strength bolts (e.g. 8.8 / 10.9 / 12.9) in structural applications.

Typical characteristics:

Manufactured from medium/high carbon or alloy steel, heat treated.

Higher hardness than standard washers (e.g. often in a range roughly equivalent to HV 200–300+, depending on standard).

Sufficient thickness and stiffness to resist indentation and deformation.

Functional behaviour:

Can withstand higher bearing pressure under the bolt head/nut.

Reduce embedment and loss of preload in heavily loaded joints.

Essential in preloaded/torque-tightened structural connections.

2) Stainless steel plain washers (A2 / A4)

Typical characteristics:

Material: A2 (304) or A4 (316) stainless steel.

Moderate hardness, good tensile and bearing strength for general use.

Excellent corrosion resistance.

Functional behaviour:

Best suited to stainless assemblies, outdoor use, food/pharma, marine (A4).

Under very high preload, they can still indent softer mating materials, so for highly stressed joints in stainless structures you may consider thicker washers or special hardened stainless washers if available.