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Form B Washers (DIN 125-B / BS 4320B)
Flat Form B washers in zinc and stainless steel for increased load spreading in clamping and spacing applications.
Form B washers are flat washers designed to spread load under a bolt head or nut across a wider surface area than standard Form A washers. Their larger outer diameter provides improved pressure distribution, helping to reduce the risk of pull-through or surface damage when tightening fixings.
These washers follow common metric standards including DIN 125-B and BS 4320B and are commonly used in construction, engineering assemblies, machinery installations and general fastening applications.
Available sizes by finish
| Washer size | Zinc Form B | Stainless Steel Form B |
|---|---|---|
| M6 | ✓ | ✓ |
| M8 | ✓ | ✓ |
| M10 | ✓ | ✓ |
| M12 | ✓ | ✓ |
| M16 | ✓ | ✕ |
| M20 | ✓ | ✕ |
How do I choose the correct washer size?
Choose a washer size that matches the bolt diameter. For example, use an M10 washer with an M10 bolt. This keeps the washer centred on the fastener and allows the load to be spread evenly across the surface.
Form B washers are typically selected when additional surface coverage is required. Because they have a larger outer diameter than Form A washers, they are often used when fixing into softer materials, oversized holes or applications where improved load spreading is beneficial.
Zinc vs stainless steel Form B washers
| Feature | Zinc Form B washers | Stainless steel Form B washers |
|---|---|---|
| Material | Carbon steel with zinc plating | A2 stainless steel |
| Corrosion resistance | Low. Best suited to clean, dry internal environments. | High. Suitable for outdoor and damp environments. |
| Typical use | Internal construction, machinery assembly and workshop use. | Outdoor construction, decking, fencing and stainless fixings. |
| Standards | BS 4320B / DIN 125-B | BS 4320B / DIN 125-B |
Zinc plated washers are typically used for general internal construction and engineering applications where exposure to moisture is limited.
Stainless steel washers provide greater corrosion resistance, making them more suitable for outdoor fixings or environments where moisture or weather exposure is present.
Frequently asked questions
What is the difference between Form A and Form B washers?
Form A washers have a standard outer diameter. Form B washers have a larger outer diameter, allowing them to distribute load across a wider surface area.
Because of this, Form B washers are often used where the fixing surface is softer, the hole is oversized or additional load spreading is required.
When should I use Form B washers instead of Form A?
Form B washers are commonly used when a larger contact area is needed under the bolt head or nut. This helps prevent the fastener from pulling into the material and improves clamping performance.
They are often used in timber construction, steel assemblies and applications where increased load distribution is beneficial.
Are stainless steel washers necessary for outdoor use?
Stainless steel washers are recommended when the fixing is exposed to moisture, weather or corrosive materials. Zinc plated washers are typically used for clean, dry internal environments.
If additional load spreading is required, consider Form C washers or penny repair washers with an even larger outer diameter.
Looking for other washer types? Explore our full washer range, including Form A, Form B, Form C, penny repair washers, spring washers and square plate washers.
