Mudguard Washers

Mudguard Washers

A mudguard washer (sometimes called a penny washer or repair washer) is a flat washer with a much larger outside diameter than a standard washer for the same bolt size. The wider footprint spreads load over a bigger area — exactly what you need when you're bolting through sheet metal, soft timber, plastic, or oversized clearance holes. AIMS Industrial stocks mudguard washers in the common metric and imperial bolt sizes, in the finishes that suit Australian conditions.

Sizes and specs

The mudguard washer range covers M5 through to M20 in metric, plus the common imperial sizes. The OD is typically two to three times that of a standard flat washer for the same bore. Standard thickness is around 1.2mm to 1.6mm depending on the size — thick enough to spread load without being unwieldy.

Where mudguard washers earn their place

• Sheet metal fastenings — bodywork, panel repairs, and ductwork where the bolt would otherwise pull through
• Timber framing and decking — coach screws and bolts where load needs spreading on a softer substrate
• Oversized clearance holes — when the hole is bigger than ideal and a standard washer won't bridge it
• Roofing and cladding — under bolt heads where the substrate is thin metal sheet
• Plastic and composite — to spread load and prevent crushing the substrate

Finishes available

• Zinc plated (ZP) — general-purpose, indoor and protected outdoor use
• Hot-dip galvanised (HDG) — outdoor and corrosive environments — the heavy-duty option
• Stainless steel 304 and 316 — for marine, chemical, and food-grade applications

Match washer to substrate, not just bolt

The point of a mudguard washer is to spread load on the substrate, so the substrate dictates the right size. Soft timber wants a bigger washer than hard pine; plastic wants a thicker washer; thin sheet metal wants a wider washer to bridge a generous clearance hole. When in doubt, go one size larger on the OD — the cost difference is negligible.

Specifications you'll see on the box

Mudguard washer specifications include bore size (e.g. M10), outside diameter (e.g. 30mm), thickness (e.g. 1.5mm), and finish. The OD-to-bore ratio is the spec that distinguishes a mudguard washer from a standard flat washer — typically 2-3x larger OD. DIN 9021 is the European standard for mudguard washers and covers most metric sizes; US specifications use SAE pattern. If your application calls out a specific standard, check before ordering.

Pack sizes

Mudguard washers are stocked in bag packs (typically 50, 100, or 250 per pack depending on size) and bulk packs (1000+) for fitout and installation use. Smaller sizes are economical in larger packs; the M16-M20 sizes are usually bought in smaller quantities as needed.

Need bulk packs or specific finishes?

contact our team — we stock the trade pack sizes and can source on bulk for fitout and installation work.