Here's Why Engine Oil Filters Are So Fascinating

Photo: Purolator

On a shelf in my garage sits a truly shocking collection of oil filters, including those for cars I don’t even own. I’m not sure I have a good explanation for my obsession, other than simply that oil filters are beautiful contraptions. Here’s why.

Before you judge me, hear me out on this—oil filters are far cooler than I bet you ever thought. There’s a crap ton of variation between brands on things like gasket material, anti drain-back valve material, media material, spring type, end-cap material, and pleating number. I think I’ve probably spent a hundred hours watching oil filter tear downs (like the one below) on YouTube.

But it’s important stuff! You can’t have small dirt particles getting caught between bearing surfaces; that’ll destroy your engine in no-time. The oil filter plays a huge role in keeping your engine from croaking after, for example, sand from a leaky air induction system gets into your precious crankcase.


How Oil Filters Work

Image: Ken Filter

Your engine’s oil pump pushes oil into the filter—which is sealed to your engine’s oil filter housing with a square-section round ring like the one shown below—through a number of radial holes in the filter’s metal “base plate.” The pressure generated by that pump is enough to push an anti-drainback valve—which sits just on the other side of the base plate—out of the way, at which point oil can enter the filter.

A Purolator PureOne filter: my go-to oil filter.

Once in the filter, the oil surrounds a cartridge, which consists of a usually-metal center tube surrounded by cellulose-based or synthetic pleated fibers, sealed off by metal, plastic or fiber end-caps. Once oil has passed through the base plate holes, through the anti drainback valve, and has surrounded the filter element, it passes through the pleated medium, through the holes in the center tube (shown below), and back up through the exit at the top of the filter.


The two most interesting parts of an oil filter are the anti-drainback valve and the bypass valve. The anti-drainback valve, shown below, makes sure the filter traps oil when the engine isn’t running. It’s essentially a nitrile or silicone one-way check-valve that prevents oil in a filter—particularly one that’s mounted sideways or upside-down—from flowing back into the sump. This means, when you come to start your engine back up, you’ll get instant oil pressure to lubricate your parts (i.e. you won’t risk a “dry start”).

Photo: cadzillals6/YouTube

The second interesting valve is the bypass valve, or release valve, whose job it is to allow oil to bypass the filter cartridge, flowing unencumbered by the filtration material. The point, here, is to ensure that high oil filter pressures—like those that might occur if the filter element has reached its capacity (like if it’s been left on a car for too long, and has clogged with dirt) or if the oil’s viscosity is too high (especially in cold conditions)—can open up a valve to allow for adequate engine lubrication under all conditions.

gif: cadzillals6/YouTube 

Also inside an oil filter are leaf or coil springs (that’s a leaf spring below, while the schematic at the top of this section shows a coil). Those springs simply act to push the cartridge up against the anti-drainback valve.

gif: cadzillals6/YouTube 

Another important thing to mention is the efficiency rating that you see on oil filter boxes (like the one shown below). That efficiency rating is simply a measure of the percentage of ~20-micron dirt particles that the filter can capture over a certain duration. You can read more about that efficiency test, called ISO 4548-12 (which is 86 minutes in total and requires both up-stream and down-stream particle counting), test here.


So those are the technical reasons why I’m so into oil filters. But there’s also a much less scientific justification: I just think they look fantastic. I mean, look at this classic Purolator red metal filter; it’s gorgeous!


If you’re yearning to learn more about how filters work, and how they’re made, Fram has two great videos on the topic on YouTube. Here they are:

And the sequel:

Surely I’ve convinced you that my oil filter thing isn’t that weird? No? Dammit.

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About the author

David Tracy

Sr. Technical Editor, Jalopnik. Always interested in hearing from auto engineers—email me. Cars: Willys CJ-2A ('48), Jeep J10 ('85), Jeep Cherokee ('79, '91, '92, '00), Jeep Grand Cherokee 5spd ('94).