Orv (orv) wrote,

Mechanic tips: Crush washers

Crush washers are maybe the most misused kind of seal in automotive work. They're commonly found any time a threaded fastener has to seal in a liquid; the most common example is the crush washer under the oil drain plug of most cars. Tighten the plug, the soft copper washer deforms to fill any gaps, making a liquid-tight seal.

What makes them misused is these are supposed to be one-time-use items. You can maybe get away with reusing them once, if you're careful; but after that, the washer gets work hardened and doesn't deform anymore. The usual response to this is to crank down on whatever it's sealing to try to stop the leak. In the case of the oil drain plug, this usually results in stripping out the threads in the oil pan.

Most parts stores have a rack of oil pan crush washers of various sizes; they're cheap, and should be replaced with every oil change. But what if you have an odd size? I was faced with this on Saturday, when I needed to replace the two crush washers that sealed a banjo fitting on the hydraulic pump for the Mercedes. They were an odd size and none of my local shops stocked anything like them.

It turns out there's a trick to re-using these things -- but only if they're copper. (If it's aluminum, you're out of luck, but copper is more common.) You have to anneal the metal to counteract the work-hardening and make it soft again. Hold the washer in a pair of pliers and heat it with a torch until it glows cherry-red. (An ordinary plumber's torch will do.) Once it cools, take a file and file down any ridges or burrs until the washer is smooth again. Now you can reuse it and it won't leak.

One more note -- you don't have to reef down on these things to get them to seal. That just risks stripped threads. With a good crush washer, tightening about a quarter turn past where you start to feel resistance will usually do the job.
Tags: tips
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