Leaking Lycs

In truth, all engines can leak. Here’s how to deal with it.


One of the best things about my little jet (with a top-mounted engine) is the complete lack of oil on the belly! I never have to worry about crawling underneath with a solvent-soaked rag to get the grit and grime off before it ruins the paint. But pilots and owners of homebuilt piston-powered aircraft everywhere (including me) go to great lengths to keep their machines’ bellies clean. Oil attracts dirt and dust from the runway to make that gooey black mess, and the more oil that comes out of the engine, the worse it gets. Of course, those with pusher configurations can laugh along with jet drivers—if there is no belly downstream of the engine, you have nothing to clean!

Oil separators are one way to reduce the mess, but many are disappointed that even after going through the effort of installing one, they still have oil on  the belly. The problem, of course, is that while an ideal engine sends oil only out the breather line, in the real world, oil can leak from all sorts of different places—despite the best efforts of design engineers and mechanics.

Coked oil on the muffler or heater shrouds is a sure sign of a leak, but it is only one clue about where it is from. Oil can come from anywhere and end up everywhere.

Lay of the Land

The typical aircraft engine—whether that’s a true-blue Lycoming, a so-called clone or even a similar large-displacement, air-cooled powerplant—might start out dry and clean, but it is not uncommon for leaks and drips to develop after a hundred or more hours. Things shift, seals take a set, and eventually you find that before you can do a decent inspection, you have to buy a jug of Stoddard solvent and a cleaning gun.

That engine cleaning is the first step in tracking down any leak. Airflow underneath a cowling goes in odd directions, which means leaks are not necessarily where you think they are. I have seen leaks from rear-mounted accessories show up as drops of oil on the starter—it’s all about how the air carries the oil around.

Oil on the sides and bottom of the sump can come from the sump seal. Make sure the bolts and nuts are torqued properly.

So clean things up—and I mean thoroughly, like you’re planning surgery in there—then go run the engine for a little while. If you think you have a big leak, run it just a short time or the leak will spread enough so that it is hard to localize. If it is a minor leak, clean it up and go fly for half an hour, then come back and de-cowl to check it out. If you started with a clean engine, the leak location should begin to give itself away.

It’s important to be systematic. If you just bop around the engine compartment randomly, you’ll probably miss your real problem. The other thing to remember is Occam’s razor, which states that in most cases the simplest explanation (or solution) is usually the correct one. Leaks either come from fittings, seams or cracks—and since cracks are comparatively rare, they should probably be the last on your list. Start with the easy stuff that is most likely, and let the big worries move to the back of your mind. You’ll see why I say this in a moment.

It is very common to have an oil drop off the bottom of the starter. There is nothing to leak there, however; airflow under the cowling just moves oil to that spot.

Start Simple

Begin your hunt for leaks at fittings—any place in the oil system that has a B-nut that you can put a wrench on. Check them all! Start at the top front of the engine and work your way down and aft—or if not in this direction at least systematically. If you have a constant-speed prop, check the governor line fitting on the nose piece, then move aft and check the oil drain-back line fittings on each cylinder head. On the back, check the prop line on the governor, then all of the hose fittings for an oil cooler (if you have one). Check the oil line that goes to the oil pressure sensor as well. In short, if it has oil in it, check for tightness!

It’s easy to tell if oil is leaking from a pipe thread fitting. This one is dirty but not oily.

A very common source of oil on these engines are the rocker-box covers. Look for drips on the bottoms of these, and check the screws for tightness while you’re there. If you still have the original cork seals, consider replacing them with the orange silicone seals. But be careful not to overtighten the screws if you do—the torque recommended for these is very low. Tighten a little, then run the engine and tighten a little more if you see drops from the bottom screw.

A huge number of leaks can be found this way—fittings can loosen over time. More likely, they weren’t tightened properly in the first place. Checking fittings for tightness should be done at every inspection—make it a habit to check them all with a wrench at every condition inspection. One other possibility to check, if you feel that a fitting is really leaking, is to remove it and look at the tubing flare to make sure it is properly done, has good contact all the way around and isn’t cracking, I have seen defects for each of these cases in flying airplanes.

Check the oil cooler fittings—both the pipe threads where elbows screw in to the cooler and the hose ends. Overtightened elbows can crack the cooler boss—undertightened ones can seep.

Now while we’re just poking around, let’s do one more thing—let’s check for a crankshaft nose seal leak. Grab a flashlight, inspection mirror and a clean paper towel. Take a look up and inside the flywheel—this is most easily done with a mirror and by looking up above the alternator, most baffles and the starter block; there’s good access anywhere else. See if you see any sign of oil in there—if so, it can only really come from one place. If you’re not sure, take that clean paper towel and wipe the inside of the flywheel—that will tell you if you have any oil in there. If so, add this to your list of things you might have to go back to.

Moving Along

Next thing to check is anything secured with a hose clamp—the couplings for the oil drain-back lines underneath the cylinders, for instance. The factory- designed clamps for these rubber tubes are worm-drive hose clamps, two for each short piece of hose. Unfortunately, the nature of a worm clamp of that size is that it will not put even clamping pressure all the way around.

Silicone rocker valve cover gaskets help keep oil in—but they leak if overtightened, so go slow on those screws.
The base of the dipstick tube is a common leak source as no one knows what seal to use—an O-ring or gasket. I’ve had the best luck with a gasket and form-a-seal plus safety wire.

Owners of Lycomings have struggled with leaks here for a long time and many are now substituting automotive-style spring clamps that are installed with pliers; they tend to evenly clamp the hose for its full circumference. It’s not factory, but your engine is on an Experimental, so if you have trouble with leaks here, the new style clamps are worth a try. Also, if the hoses are chewed up by the clamps, replace them.

Continuing our search at this level, we are basically looking for any place that a “soft seal” might be leaking. It is important to remember that Lycoming’s recommended TBO (which we are not required to observe) is generally expressed as a number of hours or a number of calendar years—2000 hours and 12 months being applicable to most of the four-cylinder, direct-drive, horizontally opposed engines. Hours in service is a criteria that is easy to understand—it is a measure of potential wear.

Pro tip: If you suddenly have a leak around the oil dipstick on a new engine, check to make sure the dipstick is tight. Yep, it’s happened. But not too tight: A loose dipstick tube will make a huge mess.

But what’s the deal on those years? Seals are soft goods that age and become stiff or brittle with that age—so a growing number of leaks in seals is an indicator that you might be ready for a teardown and rebuild. If your engine is low on hours but high in years, you might be able to get away with a simple reseal without having to do a complete overhaul. But in having it apart, I bet you’ll find corrosion (the result of few hours in a lot of years) and need to go the full overhaul route anyway. I’m not saying that you have to tear your engine down after the age requirement, by the way—I am simply saying that if you’re getting leaks with age, it might not be unexpected.

If you find coked-on oil around the lower spark plug or exhaust gasket, the most likely place to check is the outboard pushrod O-rings. These don’t last forever and are likely leak sources after 10 years or so.
Typically, you’ll want to look above for the source. For example, if an oil filter is loose enough to leak, it is really loose. Rarely is this the cause of a drip unless you have debris in the seal.

But before we entertain such a painful thought, let’s move to the back of the engine—where the accessories live. Most likely, you have a fuel pump, an oil filter (and adapter) or screen, magnetos (or ignition pickups), a prop governor and a vacuum-pump pad. Each of these has fasteners and gaskets that could leak. Use a bright flashlight to look for leaks, then put a wrench on every mounting bolt or nut and make sure everything is tight. The good news on accessories—if you find a leak from a gasket, it’s relatively easy to buy just the gasket you need, pull the device, replace the gasket and fix the problem. It is especially common to have mag gasket leaks, by the way; these often get disturbed in the process of timing the ignition and then they don’t seal.

The inner pushrod seals can leak when they get old (right). Look for oil tracks starting right at the seals as a clue. Plentiful airflow in this area can blow leaked oil everywhere!

Don’t overlook the crankcase breather line, either—usually there is a short length of rubber hose at the top and the bottom is often terminated above an exhaust pipe, plumbed into an oil separator or just vented into space. The good news is that if these are leak sources, they are generally easy to find and fix. Just tighten the appropriate clamp.

Leaks from magneto seals are common. If you have a leak on the back of the engine, check to see if these are the source.

Before we leave the area of seals, let’s go back to the top of the engine and take a look at pushrod tubes—all of them—and both ends of each. It is fairly easy to see the end that tucks into the crankcase, and a leak there—from the green seals—should be obvious. The end at the cylinder head is a little harder to see, however, and is sealed with an O-ring that can deform and leak with age. If you find caramelized oil on the exhaust port or around the lower spark plug, think of where it is coming from—the outboard end of the tubes is the likely culprit. The seals on both ends can be changed fairly easily with just a little investment of time.

Bigger Problems

If you have tightened all of the fittings and hose clamps—and still have a leak—you might have a problem that is a little more difficult to fix. The major joints on the engine include the one between the two case halves, the sump attachment plane and the accessory-case joint on the back of the engine. Add to these the base of each cylinder, and you pretty much have all the places that are sealed by gaskets of one kind or another.

Oil return-line couplings are always good places to check for leaks, especially with worm-drive clamps, which don’t seal well. Tightening them rarely makes them seal better.

If you have a leak at the base of a cylinder, oil is somehow getting past the cylinder base O-ring. The only way to fix this is to remove the jug—not a terribly difficult thing to do once you have the baffling removed. (I can see the wind go out of your sails with the mention of baffling—but sometimes you just have to bite the bullet and remove the tin.) If you have to pull a cylinder to replace the O-ring, understand that it is invasive enough that you’ll want experience on your side—either your own or from someone who has disassembled and reassembled a Lycoming before. Again, it’s not a difficult job, but one that is more involved than we’ll discuss here. When you get the cylinder off, make sure to check the mating surface on the case for scratches or nicks that might be the actual source of the leak.

If your problem is not with the cylinders but with the case itself, you can start by checking the torque on the outer case bolts (they are 1/4-inch bolts) and the bolts that hold the sump and accessory case to the engine. You’ll find both bolts and nuts on studs as you work your way around. Truthfully, tightening these bolts beyond torque value isn’t going to fix a leak very often—only if they are really loose are you going to solve your problem this way. More commonly, you are going to find a gasket that is improperly sealed or one that has finally given way to a latent problem from the build or age. The three-way joint between the case halves and the front center of the sump is a place that oil can often seep, and the only way to fix this is to take everything apart and reseal the engine properly from the start. Pro tip here: Trying to put some sort of sealant over a leaking joint is rarely going to work, and is sort of a last act of desperation. Oil will continue to leak underneath any patch and separate said patch from the engine.

It is not uncommon to find drops of oil on the fuel pump screws—but it rarely comes from the fuel pump. This oil is most likely coming from above or ahead.

The case halves of a Lycoming are traditionally sealed with a little bit of adhesive to hold two lines of silk thread in place—and that is it! Lycoming does allow a more modern silicone sealant, but most of the big engine builders I have checked with still use silk thread because it works. The sump and accessory case use gaskets, and if you use proper torque, the engine should be tight. Oh, and back to that crankshaft oil seal up front—if you have a leak from there, you’ll have to pull the prop, but the fix is within the realm of a typical homebuilder.

The Worst News

If, after checking everything we’ve talked about already, you still have a leak you cannot find, you might have a really bad problem—a crack somewhere in your case. Finding such a leak is vitally important if one exists—the last thing you want is the crankcase to fail and send parts of the engine bouncing around inside the cowling. Cracks are best found by using a dye penetration kit, and you’ll be thoroughly cleaning the engine again, then running it to try and find the seep. You might well have to remove the baffling to really see all parts of the case, so this is a pretty involved process.

The breather fitting can leak if the hose is old and gooey, so check to see if you have oil seeping out—especially if you have done any zero or negative G maneuvers.

Fortunately, it is also a rare problem—more common on an old case that has seen several overhauls—so keep things in perspective. Yes, case cracks happen, but all of the things we have talked about so far are much more common and therefore more likely. In other words—don’t panic! Take your time, completely investigate all possible sources, and get another experienced opinion (or two) before you despair.

Leaks Be Gone

No one likes a messy engine—although a typical oil leak rarely gives a measurable drop on the dipstick, it doesn’t take more than a couple of tablespoons of oil to make a real mess! There is nothing as nice as a new motor that leaks nothing at all, and the first time you take your cowl off and find spilled oil can be a real downer. But when that happens, take a look at the points we’ve outlined and stay on top of it. Fixing a small leak early is far easier than finding one on a completely oil-soaked motor and, no, spilled oil doesn’t make anything run better.

A leak around a cylinder base is rare, but when it happens, it can be a pain to fix. You have to pull the jug and check for damaged mating surfaces. It’s actually more likely the leak is coming from a pushrod tube, but it’s worth checking the torque on the cylinder base nuts.
The breather exit can send oil overboard—that is its purpose. If it is terminated in the cowling above an exhaust pipe, it’s worth checking to see if there is a lot of oil coming out.

The truth is, the basic Lycoming (or clone) design is pretty rugged and well proven—and will continue to run reliably even when it leaks. Nothing is as bad as those old radials. “Hey son, would you fill up the oil, and uh…check the fuel while you’re at it?” But it sure is nice when you can clean a few exhaust stains off the belly and not worry about where the oil is coming from. These engines are, in fact, not supposed to leak at all—so when they do, get out the flashlight and wrenches, and stop the seepage! Your plane and engine will be much happier in the long run.

Photos: Marc Cook

A spine leak between the two case halves is telling you that something is not sealed properly, and the only real solution is splitting the case and having it machined (left). You could slap it back together with new sealant, but once you have it apart, why not go the whole way?
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Paul Dye
Paul Dye, KITPLANES® Editor at Large, retired as a Lead Flight Director for NASA’s Human Space Flight program, with 40 years of aerospace experience on everything from Cubs to the Space Shuttle. An avid homebuilder, he began flying and working on airplanes as a teen and has experience with a wide range of construction techniques and materials. He flies an RV-8 and SubSonex jet that he built, an RV-3 that he built with his pilot wife, as well as a Dream Tundra and an electric Xenos motorglider they completed. Currently, they are building an F1 Rocket. A commercially licensed pilot, he has logged over 6000 hours in many different types of aircraft and is an A&P, FAA DAR, EAA Tech Counselor and Flight Advisor; he was formerly a member of the Homebuilder’s Council. He consults and collaborates in aerospace operations and flight-testing projects across the country.


  1. I took a page from the drag-racing world. Run engine until case is warm, solvent wash and the heat of the metal will dry it. Then spray the suspected areas with either Spot-Chek developer or spray foot powder, either will leave a flat-white film on the engine.
    Then run for a few seconds after the oil pressure indicates , shutdown, and check for the spot.


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