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About Oil Filters

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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.

OIL FILTER BYPASS VALVE

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.

OIL FILTER QUALITY

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.


oil filter exploded view

CHOOSING AN OIL FILTER

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

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

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.





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