Air And Oil Filters - Basic Training - MC Garage

Tips, Tweaks, Fixes And Facts

Filters-air and oil-are your engine's primary lines of defense against dirt, sand, metal particles and the like. The consequences of either line collapsing like the Maginot can be either greatly accelerated wear or a sudden, catastrophic failure. There are plenty of opportunities for those lines to get breached, too, especially the air filter's. Your motorcycle's engine is basically a glorified air pump, and it sucks in roughly enough air to fill 1800 basketballs for every gallon of fuel. "So?" you say. Well, studies suggest a cubic mile of air over a typical city contains more than 400 tons of dirt. Odds are some of that dirt is at least going to make it into your bike's airbox.

Naturally, there are different schools of thought on the precise size of particulate material that can cause damage to your engine, such as to the relatively soft aluminum of the pistons' skirts, or to the somewhat more durable rings and main bearing materials. One aftermarket firm, for instance, says engine wear is caused by particles larger that 5 microns. One study says approximately 60 percent of all engine wear is caused by dirt particles 5 to 20 microns in diameter; other research says it's 10 to 100 microns. And yet another aftermarket filter maker (and OE supplier) says contaminants of any and all size will harm your engine. Now, bacteria measure about 2 microns in size, while grains of talcum powder range from 5 to 10 microns, white blood cells measure 25 microns, a human hair is about 70 microns, and fine sand, coal and cement dust are up around 100 microns. (A micron is one-millionth of a meter, or 0.00003937 inch.)

At least we can see a pattern develop. At the low end, let's say anything over 5 microns is bad for your engine; at the high end, let's figure if a filter passes large amounts of something larger than a human hair in diameter-70 microns, which you can see with the unaided eye-there's something drastically wrong with that filter.

Another area of little consensus is the best filter medium. Typically, most OE air and oil filters tend to use some sort of treated paper-cellulose, basically-although certain manufacturers and aftermarket filter makers utilize paper hybrid media-paper with glass fibers or synthetic material. Some aftermarket firms rely solely on synthetic media for their oil filters. Oiled cotton gauze is one popular medium for aftermarket air filters; another is oiled polyurethane foam, which even OEs use on most of their dirtbikes.

Each medium has its own litany of specialties. Generally speaking-and I emphasize the word generally-oiled foam is accepted to have the best filtering capability for air filter use. When it comes to oil filters, paper, paper hybrid or synthetics are the media of choice. For original equipment manufacturers, paper or paper hybrids provide the best compromise for streetbike filters: They offer average-or-better abilities for both filtration and flow (air or oil), and they require next to no maintenance. Regardless of application, such filters are simple R&R; items-just remove and replace.

Oil filters do have a couple of added complications, namely anti-drain-back and bypass or pressure-relief valves. The anti-drain-back valve keeps oil in the filter if it's mounted horizontally or upside-down, which a) reduces the time it takes to build oil pressure on start-up and b) prevents that unfiltered oil from draining back to the sump. The bypass valve opens when oil cannot flow properly through the filter either because the oil's too cold or the filter's clogged.

Not all aftermarket oil filters have these two features, or if they do have a pressure relief valve it might be designed to open too soon or too late. Too soon, and unfiltered oil is allowed to flow through the engine unnecessarily; too late, and oil pressure might drop so low that catastrophic engine failure occurs.

Nonetheless, aftermarket filters retain their appeal. Yet for all their real or imagined advantages, it's still difficult to recommend them without reservation. As one OE rep said, "Each bike has filters designed specifically for it to suit its particular needs in terms of displacement, rpm range, physical space available and desired service interval. Each [air] filter will be sized such that by the end of its service life it will still be operating at high efficiency and with a low enough pressure drop to avoid damaging the engine or impeding its breathing.

"Your bike's air filter is part of the tuned induction system. Changing the filter changes the bike's fueling and ultimately its power. Stock air filters are the best solution simply because a lot of work has gone into making them suitable for the specific bike. By staying with stock parts, you keep what you originally bought your motorcycle for."

You might dismiss such statements as mere OE propaganda. Yet while there's a bit of truth to that harsh appraisal, there's not as much as some would like to think. Ultimately, the most important thing to remember is to follow the service recommendations for your stock or aftermarket filters, because they will eventually clog with use. And whether it's an oil or an air filter, your engine is going to come to a halt, either temporarily-or for good.