The mechanics consider a crush washer to be used once; it’s a one-and-done deal. Why? Because after you put the crush washer between the oil pan and the drain plug, you cinch down bolt. As you tighten it, the soft metal compresses itself, filling any minor irregularites in the metal faces. It gets all wonky and that’s the entire purpose of the crush washer. Once crushed, it can’t perform that task anymore.
Why does it work? The metal was manufactured in an annealed state (softened) to allow it to yield at proper torque but not crack. Here’s what happens step by step (see chart above). Before: The washer sits flat with gaps The washer is flat and has clear gaps. It starts to flow as it is tightened. Done: The correct torque allowed it to flow into all the tiny gaps, sealing it.
Why Crush Washers Are One-Time Use Only
It’s the same principle for banjo bolts on your fuel system or brake lines…those just have two washers, one on either side of that eyelet. They’re all made of different stuff. Copper’s used most often because it’s flexible and will handle greater heat. This is important for brake calipers and turbo oil feed line. On Japanese cars you’ll see aluminum washers a lot because they’re light, inexpensive and still seal well enough for normal oil drain plug use. There are steel ones too, meant for high pressure uses like hydraulics, but you have to be extra careful with tightening because the metal doesn’t flow better then it should. If you’re only dealing with low pressure, fiber or bonded washers is a reusable alternative; most mechanics still swap ’em out at regular service intervals, though.
These selections are also feasible based off the handy size chart. Small motorcycle components like M8 and M10 bolts threads in all the way to large marine and industrial diesel engine fittings like M24 and M30. Passenger car applications are pretty much M14 by 1.5; that’s for VWs, Hondas, Toyotas, BWMs…pretty much every popular vehicle. As you can see, bolt diameter directly relates to torque specs. The chart matches sizes to their torque ranges to ensure you neither crack your washer nor leave a leaky under-tightened connection.
Typically, the sins are missed details. Number 1 on the list is using your existing washer. Second is not turning the bolt by hand before using a wrench. This prevents cross-threading. Most issues are caught easy with a quick wipe of the threads and a final check the next day after your first drive.
Lesson learned? This small metal part does just one thing perfectly then gets tossed out. You should of got the torque, material and size correct and you’ve got yourself an easy oil change that doesn’t leave you wondering if you’ve done it right.
