No Muss, No Fuss Oil Changing

If you're interested in changing your own oil and oil filter -- and pilot-certificated owners are allowed to -- don't treat it as an easy job even if you've done it on your car. Complexities abound and errors are costly. AVweb presents a basic outline of the process, as well as an in-depth description of how to deal with that oil filter.


First, let’s establish the legalities here. Part 43, Appendix A of the FARs deals with preventative maintenance. Of the 30 or so listed preventative maintenance tasks, items 6 and 23 permit aircraft owners who possess at least a private pilot’s certificate or higher to change the oil and filter (or to service the oil screen) in a certificated aircraft of which they are the owner. It is not legal to change the oil in your friend’s C-150: You must be the owner (or part owner).The quality of the work must be up to same standards as would be accomplished by a certificated mechanic; i.e., FAA-approved standards using FAA/PMA-approved materials. In short, if you have never changed the oil in your plane, by all means have a mechanic take you through the steps the first time.You also have the responsibility of returning the aircraft to service by placing your name, pilot-certificate number and date of the preventative maintenance in the aircraft engine logbook. Homebuilders have no such constraints, other than that preventative maintenance may be done by the builder, current owner or a certified mechanic. That said, the FAA model is a good one.

Minimize the Mess

6)]Owners who have easily accessible filters and oil-drain plugs to oil-catch buckets may be wondering what the problem is. Those who own aircraft with arcane, inverted filter locations that hold the oil when the engine is shut down, or with devilishly located oil-drain plugs will appreciate the mess factor.I’m here to tell you that it’s possible with a few tricks and possibly an accessory or two that you can greatly reduce the mess factor. Most of the accessories are relatively cheap, with the exception of a remote-mount oil filter setup, which can cost $1500 installed. This should only be a consideration if you do not currently have an oil filter on your engine, or if it’s just about impossible to get to an existing factory setup.You may also be able to make things much easier by adding a filter adapter right to the engine to replace the existing screen. Of the bolt-on adapters, the factory model is the most expensive and least flexible, since it is straight. Aftermarket adapters are available with 45- and 90-degree bends that may make filter access a virtual breeze.Additionally, with a filter you enjoy going from 25-hour to 50-hour oil change intervals, and the comfort of superior oil filtering for potentially longer engine life. These adapters may be had for $200-400 plus easy installation by a mechanic. Aircraft Spruce & Specialty carries both the remote-mount types as well as the engine-mount types.

Low-Cost Accoutrements

Next on the list of making things easy and cleaner is the use of an oil-drain valve (also available from Spruce). This time- and mess-saver screws into the oil-drain hole and is designed and FAA approved to be just as safe as the oil-drain plug it replaces. (Non-FAA-approved models are available, too.)Warning: Be sure to check both your aircraft documentation as well as the aircraft application data on the oil quick drain. While the quick drain valves are quite short, in rare instances they can interfere with gear retraction of certain aircraft such as some Piper Arrows. (There are now low-profile units for Arrows, so be sure to check the separate catalog listing).Once you have the quick drain installed, you are free of much of the oil draining mess. For the next oil change, secure three feet of inexpensive, flexible vinyl or rubber tubing to go over the drain lip (snug fit or it can slip off at an inopportune time) and thread the vinyl tube through the bottom of the engine cowl (e.g., through the cowl flap opening) to a five-gallon bucket you have purchased expressly for this task.Be sure the bucket has a snap-on, secure lid to make oil transport to the recycler easy and clean. These are readily available at hardware and paint stores for around $5.00.Don’t forget to wait until the drain tube is in the bucket before turning on the oil flow. By the way, never drain cold oil: It does not get all the junk out of the engine nearly as well as hot oil does from a trip around the pattern. Finally, let the vinyl tube sit attached a while after the drain valve has been closed to give the residual oil in the tube time to drain into the bucket.

Completed Staff Work

If you want to do a more-thorough job, make a mid-stream catch of an oil sample, as well as drain the oil through a funnel suspended to the side of the oil catch bucket. Get a funnel that has a fine mesh screen, so as to catch any bits suspended in the oil, rather than fishing through or draining the bucket through a screen after the fact. It’s all part of a thorough, systematic job to get the maximum information that the oil change can tell you with minimum work.You can also use that same funnel to put new oil into the engine through the typically small, sometimes hard-to-reach oil/dipstick tube. Besides avoiding spilling oil onto the engine, the screen serves to avoid the not-uncommon mistake of dropping something into the engine such as oil bottle caps or the “cap keeper,” the little plastic devil that makes removing the cap so difficult. Based on complaints of low oil-pressure of unknown causes, we once tore the pan off an IO-520 and found five plastic “keepers” in the sump, lodged against the oil-sump pickup screen and blocking oil flow.Along the vein of minimal mess: Drop that old oil filter into the funnel inside the bucket so it can drain for several hours before you cut the filter open. (After an initial drain interval, you can transfer the filter to a one-pound coffee can with a plastic lid if you are in a hurry to get rid of the oil.) The next section has complete details for how to remove and install an oil filter.Be sure to not let the engine sit without a new filter for any lengthy period of time, as it is possible for some engines to lose their prime. This usually means a mechanic will need to help you to get trapped air out of the oil system so the oil will circulate properly and you will have oil pressure at startup. If you don’t have oil pressure after several seconds post-start, shut it down and seek a mechanic’s help — because you probably lost prime. Most engines do not have this potential problem.

No-Cost Helpers

Sometimes simply wrapping a half-gallon or gallon-sized plastic bag around the filter after it has been loosened just a bit to allow hand removal can significantly lessen the filter-removal mess. It takes a bit of a deft hand, but the plastic bag can catch most drips with a bit of practice. Judicious placement of shop rags is another simple preventative step. Just remember to remove them before firing up that engine!On filters that do not drain their oil contents out at shutdown, there is an inexpensive tool that punches a small hole in the filter so it can be controllably drained before actual filter removal. It’s hard to account for all the wildly weird filter placements out there.Perhaps engine designers should be forced to test their component placements for ease of maintenance before the design is signed off? Nah, too sensible.

Making the Cut

Cutting open an oil filter should be considered mandatory for the potential lifesaving information it may someday give you if you find metal or other debris. Cutting a freshly removed filter can be a truly messy affair, however, so wait for it to drain into the funnel screen first for at least an hour. Overnight is even better.Also, use a purpose-made filter cutter to save your fingers and avoid contamination of the filter contents. It also allows you do the cut in under 10 seconds. A seasoned mechanic can help you diagnose any findings. Generally you will find nothing — and that’s a very good thing, indeed. You still get peace of mind knowing that you checked an extremely important diagnostic source for the health of the engine. And it’s more thorough than most shops routinely do.

Loose Ends

Unless you are going on a cross-country flight, most engines do not need a full sump. The first quart is rapidly used/lost in most older engines. Flying a quart low is perfectly fine, and will not affect oil temperature. We tested for this problem even at minimum sump volumes. It’s not a problem.Be sure any required safety wire has been replaced, e.g., filter or oil drain plug if used. Always ground check the engine for leaks or anything left behind in the engine. Make the required log entries; and make the next flight local, if possible, so you can double-check your work and assure the filter works properly.The next section has specific details about changing the oil filter.

Oil Filter Removal/Installation

Consider the following actual incident reports:

“AC oil filter, Model OF51A, P/N25010539, installed on Lycoming TIO-540-S1 AD, was leaking at the seams and lost six quarts of oil. Total time on filter: 5 hours.”Champion aviation filter, model CH48110 (no engine or aircraft identified), spin-on type, torque nut broke from filter before recommended torque was reached. New part, original installation.”Contintental oil filter, model 642908, installed on Continental IO-520C on Beech 58, lost 3 quarts of oil in 45 minutes. The filter was ruptured at the top where the wrench cap is spot welded to the filter. Total time on the part: 20 hours.”Filter (manufacturer not identified), installed on a Continental TSIO-520-B engine on a Cessna 401A. Filter element broke up internally. Metal parts contaminated the oil system. Total time on the part: 50 hours.”

The FAA, citing these reports in a mechanics’ journal (AC43-16), commented as follows: “Problems of these types with oil filters are commonplace. All personnel involved with the manufacture, inspection, installation, and operation of these units should ascertain that their methods are not contributing to the causes.” All of this is by way of saying that oil filter maintenance — despite what you might have thought — is not something to be scoffed at or treated casually. It is very serious business. When you replace a filter — or filter element — you are performing real maintenance. (It would behoove you to perform the work under a mechanic’s s supervision.) With this in mind, let us now take a look at just what’s involved in changing an aircraft engine’s oil filter (and/or cleaning its oil screens).

Filter Types

Aircraft oil filters come in two basic types: the familiar automotive spin-on type, and a take-apart or “cartridge” style consisting of a reusable can and lid enclosing a replaceable paper element. The cartridge style means more work, and more chances for leakage.Both of these filter types — in turn — come in a short version and a long version. Which of the two versions your engine has depends, for the most part, on how much clearance exists between your engine’s accessory case and the firewall. The manufacturers generally try to install long-version filters whenever possible, since these filters (due to their larger size) offer greater filtration capacity, and proportionally longer oil-change intervals than their smaller counterparts.The foregoing filter types are often referred to as “full-flow” oil filters, which is simply a short way of saying that the filter element is mounted in series with, and immediately downstream of, the engine oil pump, so that all circulating oil must (after it passes through the oil pump) go through the filter. To guard against the possibility of oil starvation downstream of the filter in the event of filter clogging, all full-flow systems incorporate a pressure-relief valve — either in the oil filter itself or in the filter mounting adapter — to allow oil to detour around the filter element under conditions of high oil pressure.The alternative to the full-flow lubrication system is something known as a bypass filter system, in which the oil filter is mounted in parallel with the engine oil pump, rather than in series. In a typical bypass system, only about 10% of all oil coming from the oil pump gets diverted to the oil filter; the remainder of the oil goes directly to the engine’s bearings and oil galleries. (A chunk of metal or dirt thus might travel many times around the engine’s oil system before being caught by the oil filter, in this system.) Bypass systems are often used with special filters that tend to restrict flow and that, therefore, aren’t well-suited to use in a full-flow system. Very few aircraft engines employ bypass filtration of crankcase oil.

Oil Screens

2)]Before we get into the actual “here’s how” of oil-filter maintenance, it’s important to say a few words about oil screens. All present-generation aircraft piston engines have at least one main oil screen (a relatively coarse metal screen designed to remove large impurities from circulating oil) in the line leading from the oil sump or oil tank to the oil pump. This is called the suction screen — so named because the screen is located just upstream of the oil pump. On Lycoming engines, the suction screen is usually found at the bottom-aft end of the oil sump, installed horizontally, although on a few models the screen is installed vertically near the carburetor. (Have a mechanic point it out to you if there’s any doubt in your mind where to find the suction screen on your engine.) On most Continental engines, the suction screen is located deep inside the sump and does not come out for servicing, except at overhaul time. (An exception to this is the Continental 0-300 series engine, which does have a removable, horizontally mounted suction screen.)Along with the suction screen, most engines also have a pressure screen, located just downstream of the oil pump. The only engines that don’t have a pressure screen are those that employ a full-flow oil filter, in which case the oil filter itself substitutes for the screen. Depending on what type of engine you have, then, it’s possible that you could have either: (1) a suction screen and a pressure screen, both removable for servicing; (2) a suction screen and a pressure screen, with only the latter removable; (3) a removable suction screen and full-flow oil filter; or (4) a non-removable suction screen and a full-flow oil filter. In any case, if you do have a removable screen (or screens), you should plan on inspecting and cleaning the screens at regular intervals, preferably at each oil change. If you have a full-flow oil filter, you should probably plan on replacing it at 50-hour intervals, or however often your aircraft service manual suggests.Servicing an oil screen (either suction or pressure) rarely takes more than couple of minutes. Assuming you’ve already let the oil drain out of your engine, simply cut the safety wire on the plug hex (noting, for future reference, exactly how the plug was wired to begin with), unscrew it from the sump (or the engine accessory case, if it’s a pressure screen), and carefully pull the plug straight out of the engine. A small, cylindrical screen will come out with the plug; this is what you’re after.Examine the screen thoroughly. In all likelihood, you’ll see a few tiny slivers of metal, along with some carbon; this is normal. (If the engine has been overhauled in the past 100 hours, it is not unusual to see as much as 1 /4 teaspoon of “sump solids” in the screen at each oil change.) If you should discover a lot of metal in your oil screen, don’t panic; but by all means do send the fragments to the engine manufacturer for analysis. Both Lycoming and Continental have their own metallurgical labs and are happy to analyze any screen trappings their customers care to send in.After inspecting your oil screen, wash it in naptha or Varsol (or gasoline, etc.), dry it with compressed air, and — after checking the strainer cavity in the engine with your finger to be sure there is no debris there — replace the screen in the engine, using a new gasket. (Your FBO should have plenty of these gaskets on hand.) The screen should go almost all the way in, using finger pressure only. If you find you need a wrench to tighten it down immediately after starting the plug, the screen may be misaligned and you should recheck it.Finally, safety-wire the plug with MS20995-C41 lock wire. Be sure the wire wraps around the plug hex in a tightening direction.

Changing Filters: Cartridge Type

Changing an oil filter can either be quick and easy or unbelievably exasperating, depending on what kind of filter assembly you have (spin-on or can/stud/cartridge), where it’s located, and whether or not the filter is stuck. If you have an older-style can/stud/cartridge type of oil filter, replacement of the paper element can be accomplished as follows:

  1. After gaining access to the filter assembly, remove all safety wire from the can and stud (noting, for future reference, how the wire was installed).
  2. Unscrew the hollow stud, using a 1-inch, open-end wrench or a socket wrench with a crow’s foot adapter. (It’s a good idea to place an old coffee can under the filter at this point, to catch the drippings.) Remove the filter assembly from the mounting pad as a unit.
  3. Invert the filter assembly. Discard the filter-to-engine gasket. Unscrew the nylon nut from the threaded end of the stud and set the nut aside; then press downward on the end of the stud, pushing it out of the filter can.
  4. Lift the lid or cap off the filter can, exposing the filter element. Throw away the gasket between lid and element.
  5. Remove the old filter element from the can. (Don’t throw it away yet; you’ll want to open it up later to look for metal chips.)
  6. Clean the stud, can, and lid in gasoline, then wipe dry with a clean rag. Set the can in an upright position on a perfectly flat surface to check for distortion; if an out-of-flat condition greater than 0.010-inch exists at the rim, replace the can.
  7. Lubricate the new copper washer, the two new rubber gaskets, and both ends of the replacement filter with clean engine oil . (Dry gaskets will cause erroneous torque readings in Step 11.) Do not use any other lubricants.
  8. Place new element inside can; place new copper gasket on stud; then insert the stud through can and element.
  9. Install new gasket inside flange of lid. Place the lid carefully on the end of the can and secure it there by screwing the nylon nut onto the stud until the nut contacts the lid. Use finger pressure only; the fiber nut is easily stripped if over-tightened. Be sure the nut lies flush with (or slightly below) the lid’s rim.
  10. Install new, oiled gasket on outer surface of lid. Inspect the mounting-pad gasket seat for gouges, deep scratches, etc. If surface imperfections are noted, consult a mechanic; replacement of the filter adapter (mounting pad) may be required.
  11. Install the filter assembly on the adapter. (Rotate the can so that the safety-wire tabs are in the proper orientation for final rewiring.) Holding the can to prevent it from turning, tighten the stud to the NOTI torque specified on the side of, the can (or in the aircraft service manual).


  12. Service the engine with the appropriate grade and amount of oil. Because the filter housing holds approximately one quart of oil, the engine will require at least one additional quart whenever the filter is changed.
  13. Start the engine; check for proper oil pressure-pad junction and around the stud hex at the top of the filter housing. If any components of the take-apart cartridge leak, repeat Steps 7 through 13 using fresh gaskets.
  14. With engine stopped, recheck the filter-stud torque. Check the security of the adapter to the engine: If adapter can be rotated by hand, disassemble it from the engine to check for damage.
  15. Finally, safety-wire the stud to the can — and the can to the engine adapter — in accordance with service manual recommendations, and make your log entry.

As you can see, the replacement of a can/stud/cartridge oil filter element can hardly be considered a simple, five-minute affair. A great deal of time is spent assembling, disassembling, cleaning, and inspecting parts, and once the parts are put back on the engine, they must be checked rather carefully for leakage. (Cannister-type oil filters are well-known, and widely cursed, for their ability to leak oil. Little wonder that Detroit abandoned the use of take-apart filters on automobiles years ago.)

Changing Filters: Spin-on Type

Spin-on oil filters are removed and installed in much the same way on aircraft engines as they are on automobile engines; however, not all aviation shin-on filters are designed the same as automotive spin-ons. Aircraft spin-on filters come in long and short versions, with and without a male thread in the end, as well as with and without an internal bypass (pressure relief) valve. Thus, it is extremely important that you obtain the correct part-number spin-on filter for your plane’s engine; not just any filter will do. (Consult the filter manufacturers’ application charts and your aircraft parts manual prior to purchasing a replacement filter. Don’t simply replace your existing filter with one just like it unless you know that the old one has the correct part number.)Before removing your old spin-on filter from the engine, study the installation to see how the filter is safety-wired. Notice, in particular, where the wire attaches to the engine (or the adapter). There’s a good reason why the filter is wired the way it is (i.e., to a point on or near the engine itself): namely, if it were wired to anything not directly attached to the engine, the violent shaking of the engine with respect to its surroundings during startup and shutdown would cause the safety wire to snap instantly,Note, also, whether the filter is mounted in such a way that it points up, down, or straight out horizontal. If the filter is angled upward, you can count on its releasing about a quart of oil as you begin to remove it; consequently, you should place an old coffee can underneath the filter mounting pad to catch the spillage. If the filter is mounted horizontally, you can expect about a half a quart of oil to come out as you begin unscrewing the filter. If the can is nearly inverted, most of the oil will, of course, stay in the filter and you’ll be able to keep things clean with a well-placed rag or two.

0)]There’s not much to say about the actual removal and installation of aircraft spin-on oil filters, except that if the filter is stuck to the mounting pad (as is often the case) you may well invent a few never-before-heard expletives before getting it off. Fortunately, most of the spin-on oil filters in use in aviation today incorporate a 1-inch, steel hex on the top of the can, so that both a conventional wrench and an automotive-style oil-filter wrench can be used simultaneously to free a stuck filter — if access to the filter is good enough to permit the use of such implements. (It is not unheard of for vacuum pumps and other accessories to have to be removed from the engine in order to gain sufficient access to the oil filter, in rare cases.)After you’ve removed the old oil filter, it is a good idea — especially if the filter did not come off easily — to inspect both the filter and the mounting adapter to see that the male-threaded stud is in good shape and in its proper location. It is not unusual for male-threaded filters (such as the Champion CH481 10) to leave their stud in the mounting adapter upon removal, or for the reverse to happen with fernale threaded filters. In either case, the stud should be returned to its original location, barring damage to the threads.To prevent sticking of the new filter, prelubricate its gasket not with engine oil (as is usually done), but with Dow Corning DC-4 Compound, applied sparingly but evenly. Unlike engine oil, DC-4 will not break down at high operating temperatures (thus cementing the filter to the pad). Champion specifically approves the use of Dow Corning DC-4 as an “anti-stick” agent for spin-on filters; however, Champion recommends that this compound not be used on the gaskets that accompany take-apart (can/stud-type) filters.To install a new spin-on filter, simply lubricate the filter gasket (as explained above), wipe the mounting adapter surface off with a clean rag, screw the new filter on by hand until the gasket contacts the mounting adapter, then torque the filter to 18 foot-pounds (or to the torque limits given in the instructions that came with the filter). In any case, do not tighten the filter more than 1-3/4 turns after the gasket has contacted the mounting pad. If you do, you may never get it off again.Finally, run the engine a few minutes, check for leaks, and safety-wire the filter in place. (Don’t forget to add an additional quart of oil to the engine, if need be.) That’s all there is to it.Obviously, the replacement of a spin-on type oil filter entails considerably fewer steps (and thus offers significantly less opportunity for error) than the replacement of a can/stud/cartridge type filter. For this reason alone, any aircraft owner-operator who intends to perform his or her own oil filter changes on a regular basis ought to convert over to spin-on filters as soon as possible, if the aircraft is not already using spin-on filters. (Some spin-on filters can be used as direct replacements for some take-apart filters; it’s possible you can switch over without buying any new mounting hardware at all.)It should be mentioned that spin-on filters are not only easier to install than their can/stud/cartridge counterparts, but are less apt to leak and weigh about a pound less, thus contributing to reliability. (Any weight reduction on a right-angle adapter of any kind increases its reliability.) When it comes to simplicity, reliability, and ease of maintenance, spin-on filters are without peer.

8)]Regardless of whether you end up replacing spin-on type filters or disposable paper elements, it is important that you save the old element at each filter change and open it up for closer examination, to determine whether your engine is shedding metal. What shows up in the filter may not show up in oil analysis, and vice versa.

Seven Common Oil Filter Mistakes

Oil-filter replacement may seem to you to be among the easiest preventive-maintenance tasks for a pilot/owner to accomplish — and it may well be — but many mechanics don’t see it that way. “I’ve seen guys make the darndest mistakes,” one A&P told us, shaking his head. Mistakes like these, for example:

Mistake 1: Not inspecting the new element (and mounting pad) thoroughly before installation. The FAA’s service difficulty computer file contains numerous reports of cracks, gashes, warps, loose gaskets, and other defects in otherwise-normal-looking, factory-fresh oil filters. Therefore, don’t assume that the new filter you’re installing is serviceable; look at it. And inspect the filter mounting pad for discrepancies, while you’re at it. A warped, cracked, or dented pad or adapter should be brought to the attention of a mechanic at once.Mistake 2: Not inspecting the old filter element. How do you know the old element isn’t filled with iron filings? Or oil pump gear teeth? Always cut open the old filter and examine the contents. (Detailed “sump solids” analysis guidelines can be found in Light Plane Maintenance, March 2006.) Borrow your mechanic’s can-cutter, if need be … but get that filter open.Mistake 3: Not changing filters at every oil change. Look: Your old oil filter contains almost a quart of dirty oil. Why replace you crankcase oil with clean lubricant, only to contaminate it right away with a quart of old oil? (The anti-drainback feature of your filter keep old oil from draining out of it during an oil change.) If clean oil is the objective, start with a fresh filter at each change.Mistake 4: Improperly lubricating the new filter gasket. Lubricate new spin-on filter gaskets with Dow Corning DC-4; lube take-apart filter gaskets with clean engine oil. (DC-4 is approved for spin-on filters only. In addition to providing an oil-tight seal, DC-4 will prevent filter sticking.) Fingers are the best tool here. Don’t be sparing.Mistake 5: Overtightening the filter. Always follow the filter manufacturer’s exact instructions. Spin-ons are usually torqued to 18 ft-lbs, or spun no more than 1-3/4 turns after contacting the pad. With take-aparts, the proper procedure is to hold the canister still while torquing the stud to 20 or 25 ft-lbs. Don’t exceed these limits; you may never get the filter off again.Mistake 6: Not running the engine up and looking for leaks before safety-wiring everything. The only way to know whether your new filter is on tight enough and otherwise airworthy is, of course, to run the engine up on the ground and then look for leaks. Standard procedure here is to leave the filter unwired until you’re sure everything is OK, then wire it in place.Mistake 7: Improper safety-wiring (or, worse yet, no safety-wiring.) All filters should be safety-wired after installation, using heavy-gauge (0.040-inch minimum) MS20995 stainless-steel wire. The lockwire should have six to 12 twists per inch, with no slack in the run. All runs should be in a tightening direction. Very important: The filter must be wired to a point on the engine or mounting adapter. The reason for this is that the engine shakes quite a bit compared to its surroundings, but at least the filter shakes with it. Wire the filter to a stationary point not connected with the engine, and you’ll see the wire snap like string the first time you start the engine. Tip: We find AC’s spin-on filters easiest to safety-wire, since they are wired at the bottom. Champion puts its tangs way up at the top; for those filters, insert your safety wire through the anchor point (on the engine adapter) before screwing the filter on. Access to the anchor point will be much poorer with the filter in the way.

And don’t forget to add an extra quart of oil to your engine after the filter change.

For more about maintaining general aviation aircraft, check out AVweb’s Maintenance section.