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What Ev ery Motorist Should Know About Oil Filters

Copyright AA1Car

oil filter

The oil filter is the engine's main line of defense against abrasion and the premature wear. The oil filter removes solid contaminants such as dirt, carbon and metal particles from the oil before they can damage bearing, journal and cylinder wall surfaces in the engine. The more dirt and other contaminants the filter can trap and hold, the better.

In today's engines, all the oil that is picked up by the oil pump is routed through the oil filter before it goes to the crankshaft bearings, cam bearings and valvetrain. This is called "full-flow" filtration. It is an efficient way of removing contaminants and it assures only filtered oil is supplied to the engine. In time, though, accumulated dirt and debris trapped by the oil filter begin to obstruct the flow of oil.

The oil filter should be changed before it becomes clogged. Since there is no way to tell how dirty the filter actually is, the filter should be changed according to the maintenance schedule in your vehicle owners manual, or when the oil is changed.

Though some vehicle owner's manuals say oil filter replacement at every other oil change is acceptable, most professional technicians always change the oil filter at every oil change. Why contaminate the clean oil with up to a quart of dirty oil? And why risk expensive engine damage if the old filter is on the verge of clogging up? A new oil filter is cheap insurance against premature engine wear.


If you wait too long to change the oil and oil filter, there is a danger that the oil filter might become plugged. To prevent a plugged oil filter from starving the engine for lubrication, oil filters have a built-in safety device called a "bypass valve." When the differential pressure across the oil filter element exceeds a predetermined value (which varies depending on the engine application), the bypass valve opens so oil can continue to flow to the engine. But when the bypass valve is open, no filtration occurs.

The bypass valve also opens when a cold engine is first started. Cold oil can be fairly thick and may not pass through the filter element very easily. So the bypass valve opens and allows the oil to go around the filter until the oil warms up and flows more easily. During this time, any contaminants that are in the crankcase may be sucked up through the oil pump and bypass the filter, causing increased engine wear and possibly engine damage. Once the oil gets warm and the bypass valve closes, oil flows through the filter and normal filtration resumes.


Most oil filters look alike on the outside. One might conclude, therefore, that all oil filters are the same in their filtering capacity and ability to protect the engine from wear particles. But all oil filters are NOT all the same. There can be significant differences that affect both filtration efficiency and longevity.

Some oil filters may contain up to 50% more square inches of filter media than a cheaply made oil filter for the same engine application! Which oil filter would you rather have on your engine? One with increased filtering capacity or one that may not go the distance?

The ability to trap and hold contaminants is another difference between filters. Oil filters with resin impregnated pleated paper (cellulose) elements are good at removing particles down to about 25 to 35 microns in size. One micron equals a millionth of a meter or 0.000039 in. By comparison, a human hair is about 60 microns in diameter! Yet according to some filter manufacturers, particles as small as 10 to 20 microns can also cause damage over time. To trap these smaller particles, many oil filters now use a synthetic filter media (like synthetic glass fiber), or a media that blends synthetic glass or rayon "microfibers" with cellulose fibers to increase the filter's ability to trap small particles.

Contaminants that are too large to pass through the microscopic pores between the fibers become trapped in the oil filter. As the contaminants build up, they actually increase the filtering efficiency of the media. But after a time the accumulated debris also begins to restrict the flow of oil through the filter. Consequently, an oil filter should have plenty of dirt holding capacity, too. Some "long life" oil filters can hold up to 40% more dirt than a standard oil filter.

Watch Out for Debris Inside New Oil Filters! This should NEVER happen, but sometimes you'll find loose debris such as metal particles or bits of filter media loose inside a brand new filter. Turn the filter upside down and shake it hard a few times or bang it on a table to dislodge any junk that might be inside the filter BEFORE you install it on your vehicle. Debris inside a filter will pass through into the engine's oil system and may jam the oil pump pressure relief valve open causing a loss of oil pressure. Debris can also plug oil passages to the crankshaft or cam bearings, lifters or pushrods, resulting in expensive engine damage.

Watch Out for Fake Filters!

Watch out for fake or counterfeit oil filters that are knock-offs of name brand products. These are often sold online at low "discount" prices, but they are no bargain! The filter may be packaged in a box with a name brand on it (which is counterfeit) or painted and labeled to look like the genuine product (which it is not). The filter media inside these fake filters is typically a low quality pleated paper cellulose (not synthetic fiber or synthetic media) that cannot provide the longevity or filtering efficiency of an OEM filter or quality aftermarket name brand replacement filter. Some of these knock-off filters may also be missing a required anti-drainback valve, or the pressure relief valve may open at too low of a pressure for the application. Some filters also use thinner gauge steel for the canister housing, which may leak or blow out causing a catastrophic loss of oil pressure!.


Oil filters are a high volume mass produced product, so it's not unusual that an occasional "bad" filter comes off the assembly line. It can happen with ANY brand of filter, but there is more of a risk with cheaply made "economy" filters. By bad, we mean the filter may not be assembled correctly internally (the ends of the filter media are not completely crimped or sealed at both ends inside in the can), or the oil pressure bypass valve is defective and is leaking, or there is some residual loose filter media inside the can, or the can itself leaks.If the filter fails to route all of the oil through the filter media because of internal leakage, it won't do a very good job keeping the oil clean. It may allow debris to pass through the filter that can damage the bearings.

If the internal pressure relief valve is not holding pressure because of damage or misalignment, unfiltered oil may bypass the filter element. Dirt or debris in the oil can cause bearing wear and damage.

If there is loose filter material inside the can, it will be flushed out and into the engine's oil system. If the material ends up in the crankcase, it will be draw back into the oil pump. The material is soft so it won't damage the pump gears, but it may jam open the pump's pressure relief valve causing a drastic drop in oil pressure.

Finally, if a spin-on filter canister leaks because the end cap does not seal tightly against the can, loss of oil will create a mess and eventually a loss of oil pressure (and engine damage) if the oil level gets too low.

oil filter exploded view


Follow the filter supplier application catalog to find the right filter for your engine. Oil filters that appear to be the same may in fact have a different thread size or internal valving. The hole in the bottom of a spin-on filter must be the same diameter as the original and have the same type of threads (SAE or metric). If the hole size or threads are different, the oil filter may not fit properly, leak or damage the mounting. The gasket must also be in the same location to seal against the engine. If the diameter of the gasket is too large or too small, it may leak.

Replacement oil filters should have the same internal valving as the original. Many overhead cam engines require an "anti-drainback" valve inside the filter to prevent oil from draining out of the filter when the engine is shut off. This allows oil pressure to reach critical engine parts more quickly when the engine is restarted. Oil filters that are mounted sideways on the engine typically require an anti-drainback valve.

spin-on and cartridge style oil filters


Most engines use a spin-on style oil filter with a filter element mounted inside a metal can. Fram was the first to add a non-slip coating on the outside of their spin-on filters, which makes makes removal and installation much, much easier, especially if your hands are greasy. Other filter manufacturers are now copying this approach and applying non-slip coatings to their spin-on filters.

Spin-on oil filters are great, but some newer engines have gone back to a cartridge style oil filter that dates back to the 1950s. Eliminating the exterior can reduces the cost of the filter and saves steel. But it also makes the filter much messier to change in my opinion. The filter cartridge is located inside a metal housing with a top that must be unscrewed to replace the cartridge. When you pull the filter cartridge out, it will drip oil all over, so have a rag handy to catch the drips and to clean up the mess when you change one of these. The old filter can be dropped into a plastic bag and sealed for disposal. The cartridge style oil filter housing usually has a large o-ring. The o-ring should also be replaced when the filter is changed. Make sure this o-ring is properly located and seated otherwise the oil filter housing may leak when the engine is started.

oil filter wrenches Assorted oil filter wrenches


For changing oil, you will need a filter wrench that fits the filter on your engine. If you own more than one vehicle, you will probably need an assortment of different oil filter wrenches to fit different sized oil filters. Filter diameters range in size, so you may need a small, medium and/or large size wrench to grip a range of filter sizes. Some filter wrenches have an adjustable rubber strap that can fit almost any sized filter. The rubber band also tends to grip better and slip less than a metal strap style filter wrench.

However, some filters can be very difficult to reach with a strap-style oil filter wrench because of their location. For hard-to-reach applications, a cup-shaped wrench that fits over the end of the filter usually works well. The wrench grips the flutes on the end of the filter to spin it loose.

Many late model cars now have drop-in cartridge style oil filters. The filter element is located inside a plastic housing that has a large hex nut on top. You unscrew the cap to replace the filter inside. Because the housing and cap are plastic, do NOT use too much force when unscrewing or tightening the cap because you may damage the cap. Also, use a 6-point hex socket or wrench to remove and tighten the cap. Using a 12-point socket increases the risk of damaging the cap because the socket wrench might slip and round off the hex on the soft plastic cap.

Another issue with the drop-in cartridge style oil filters is accessibility on some vehicles. On some applications (see list below), the filter housing is hard to reach. For these applications, using a special OFFSET HEX WRENCH can make the job much easier. We don't know why the auto makers failed to standardize the hex size on the filter caps so you will have to use a socket or offset hex wrench that matches your filter housing.

Lisle offset oil filter werench
Lisle offset hex wrench for hard-to-reach oil filter caps

Hard to Change Oil Filter Cartridges

Here is a list of some of the applications where using an offset hex wrench for the oil filter cap will make the job easier:

Note: The part number listed for the application is for a Lisle Tool hex head offset wrench.

40610 - 24mm for Cadillac and some BMW

40620 - 25mm for Fleetguard Filter on Motorhomes, Buses, and Some Medium Size Trucks with Cummins Engine. Also Some K&N Filters

40630 - 27mm for 2003 and later Mercedes 1.8L, Smart and some VW

40640 - 28mm for 2010-2018, 6.7L Dodge Cummins Fuel Filter Canister

40650 - 29mm (1-1/8") for Dodge Cummins Fuel Filter Housing

40660 - 32mm (1-1/4") for GM Ecotec, 2.0L, 2.2L and 2.4L Four Cylinder Oil Filter Housing; Some Saturn & Saab

40670 - 36mm for Ford 6.0L Diesel Fuel and Oil Filter Housings, Mini Cooper and some BMW Oil Filter Housings, and Mack Truck Oil Filter Housing

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