Metal-to-metal joints on drain plugs will leak fluid due to tiny scratches in the surfaces. Crush washers solve this by filling the gaps. Made of soft copper or aluminum, the washer deforms and molds to the shape of the plug and pan as you torque the bolt. This creates a cheap and reliable seal. They are used on oil pans, transmission plugs, and brake banjo fittings. You must replace the crush washer at every fluid change. Reusing a flattened washer will result in slow drips.
Mechanic Glossary
Crush Washer
A soft metal washer (typically copper or aluminum) that deforms when torqued to create a liquid-tight seal on drain plugs and fluid lines.
Frequently Asked Questions
No. Once compressed, the metal hardens and will not deform again to seal minor imperfections. Always use a new one.
Copper is softer and seals better at lower torque, while aluminum is used on aluminum pans to prevent galvanic corrosion.
If one side is flat and the other is rounded, place the flat side against the oil pan and the rounded side against the bolt head.
You can split the washer or strip the threads on the oil pan. Always follow the manufacturer's torque specifications.