Understanding your oil filter

We all know that we should change our oil and oil filter regularly. And there are plenty of discussions online about the correct oil to use. But what about the oil filter? There are plenty of brands out there, and prices vary from a couple of bucks to more than 20 dollars per filter. What’s the difference, anyway?
Well, let’s start with what’s inside an oil filter. For most all autos, trucks and motorcycles, the physical details of a spin-on filter are virtually the same. The filter is inside a Case, typically steel or aluminum. There is a Filter Media that does the real work of removing particulates from the oil. That media is bound with an End Cap at each end, and it has a perforated Center Tube. And, the filter contains two valves: an Anti-Drain Back Valve, and a Pressure Relief Valve. Not shown is the Sealing Gasket, which would be on the bottom of the filter in this illustration, and the Pressure Relief Spring, which is typically at the sealed end of the filter case. Oil is pumped into the base of the filter, on the outside of the Filter Media. The oil pressure forces the oil through the Filter Media, (hopefully) removing impurities, and the oil then exits the filter through the Center Tube and back into the engine. The Filter Media is typically a type of paper, and the End Caps are typically (but not always) steel. Some high-end oil filters use a Filter Media laminated from a combination of layers of paper, cellulose and fiberglass. The goal of the different layers is to trap larger particles in the more coarse outer layers, and finer particles in the more dense inner layers.
Now, about those valves. When the engine is running, the oil pressure keeps oil flowing into the filter, through the Filter Media, and back into the engine. Some of the impurities are trapped within the pores of the Filter Media, while other larger impurities are held against the Filter Media by the oil pressure. But when the engine stops, oil flow stops, and the pressure is eliminated. Any impurities that were held against the Filter Media by the oil pressure will drop off the Filter Media, and drift down to the bottom of the Case. At the same time, the oil in the filter will also drain down to the lowest possible point. The Anti-Drain Back Valve is there to keep unfiltered oil (on the outside of the Filter Media) and any particulates in that oil from draining back into the engine. This is very important when the oil filter is not mounted vertically, with the inlet at the top. Gold Wings mount their oil filter horizontally. That means that when the engine stops, all the large debris trapped against the Filter Media drops down to the bottom of the case, outside of the Filter Media. If there is no seal between the filter’s pile of debris and the engine, that debris can flow back into the engine as soon as the oil pressure drops. The Anti-Drain-Back Valve prevents this. It is made of a flexible material; some are silicone, while others are rubber. When the oil pressure drops, the Anti-Drain-Back Valve presses against the openings in the filter case, sealing them off, and preventing unfiltered oil and debris from flowing back into the engine.
The Pressure Relief Valve is even more critical. As the oil is pumped into the filter, it is under significant pressure. According to the 2012 Honda Service Manual, the pressure at 5,000 rpm is 77 psi. At lower rpm, the pressure is significantly less. When your engine is cold, the oil is thicker, and there is more pressure at any given rpm than when the oil is hot (and therefore thinner, or higher viscosity). The oil filter’s Pressure Relief Valve is designed to open when the pressure in the filter gets too high. Higher oil pressure can happen when the oil is cold and thick, as it is right after a cold start. This should no longer happen once the oil warms up. The Pressure Relief Valve can also open if the Filter Media becomes too clogged with removed particulates to allow the oil to pass through fast enough. It could also open when the engine rpm and therefore oil pressure are much higher, and the oil must circulate faster than the Filter Media can allow the oil to pass through.
But, if the oil filter has a Pressure Relief Valve that is not strong enough, it can open too soon, or too often, allowing unfiltered oil to circulate through the engine when it shouldn’t. The Pressure Relief Valve is typically a spring in the top of the case. When the oil flows into the filter, it presses against the base of the filter Media’s End Cap. When the pressure against the end cap is less than the spring tension, the oil flows around the End Cap, between the Case and the Filter Media, then through the Filter Media, and out the Center Tube. But when the pressure is more than the tension of the Pressure Relief Valve spring, it compresses the spring. That allows the Filter Media to be pressed away from the base of the oil filter, and the oil can them return directly to the engine.
Now, as to filter quality (and cost). There is an abundance of information on the web about oil filters. Try searching YouTube for oil filter comparisons, and oil filter cutaways, and you’ll see quite a list. The primary difference between the lowest cost filter and highest cost filters is the filter media and the end caps. FRAM filters use a cardboard end cap, while most all other filter manufacturers use steel. While I could find speculation about the cardboard end caps breaking down under heat and pressure, I could find no documented failures of this type. If the cardboard end caps did indeed fail, the result would not only be unfiltered oil being recirculated through the engine, but the cardboard fragments from the failed end cap could also be circulating with the oil. This could easily clog oil passages, resulting in catastrophic failures.
The other issue I see with FRAM filters is the comparatively smaller amount of Filter Media inside the filter. My local auto parts providers did not have a FRAM oil filter for a GL1800 in stock, so this cut up FRAM filter is somewhat larger than the correct GL1800 filter. The correct FRAM filter does have fewer pleats of Filter Media than the comparable Honda filter. Logically, less Filter Media means that it would take less sediment to clog the Filter Media surface. Technical reports and cutaway views indicate that some filters use rubber for the Anti-Drain Back Valve, which could (reportedly) breakdown or warp after a few heat cycles. Other manufacturers use silicone membranes, reported to maintain their shape and flexibility for extended usage periods.
I have cut apart a new FRAM filter and a Honda factory filter I had just removed from my bike. The Honda does have steel End Caps, and the FRAM filter does have cardboard End Caps. A popular aftermarket filter for the GL1800 is the BikeMaster 17-1608. I cut apart a BikeMaster I removed from my bike. I found it has steel end caps, and an Anti-Drain Back valve that appears to be silicone (but is clearly not rubber). The Pressure Relief Valve is implemented as a spring that holds the cartridge against the inlet (as are most other oil filters). And, it has a pretty densely packed Filter Media, so I would have no reservations about continuing to use them. And, for anyone interested enough to follow up by cutting apart a filter themselves, be advised that when done by hand, it is a real chore!
One other item I’ll mention here is that while performing the first oil change on my brand new GL1800, I replaced the stock Honda drain plug with a magnetic one from www.drainplugmagnets.com. When I next changed my oil, I found quite a bit of material clinging to the drain plug. Wiping it off on a paper towel revealed a quantity of very fine steel particles. This is actually normal for an engine, and more so for a brand-new engine, but the magnet in the drain plug trapped those particles, and made them easy to remove when I next changed the oil. How many of them would have been caught by the oil filter I’ll never know, but I do know for certain that they are no longer circulating through my engine.
If you do your own oil changes, remember these important facts from the Honda Service Manual:

• Warm the engine slightly before performing an oil change.
• The washer on the oil drain plug is a crush washer and should be replaced at every oil change.
• The oil drain plug should be tightened to a torque level of 25 foot-pounds.
• The oil filter should be tightened to 19 foot-pounds. (I have no idea how you would get a torque wrench into the extremely tight space where the oil filter of a Gold Wing is located, but there is a specified torque value for tightening it.)
• When installing the new oil filter, apply clean engine oil to the threads and the rubber seal on the outside of the oil filter case.
• Please dispose of used engine oil properly.

There you have it: most likely way more information about oil filters than you ever wanted to know. But now that you do know, the choice is yours.


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