Firewall Forward: Oil Coolers and Oil Lines

If your oil “loses its cool,” the rest of the engine is sure to follow.



Sizing the oil cooler properly and installing it so that adequate air flows through are some of your more important firewall-forward tasks. If these are done well, your oil temperature should remain between 165° and 200° in all but the most difficult hot-weather climb situations. It is important to keep in mind that the actual oil temperature, at its highest, is about 50° hotter than the gauge shows because of where the temperature sender is located. This means that a gauge reading of 165° to 200° can equate to an actual temperature of 215° to 250° at the hottest point in the engine. These numbers are important because the oil needs to be hot enough to boil off moisture that may accumulate, but not so hot that it begins to lose effectiveness as a lubricant. A well-designed oil-cooler system will work within these limits.

Oil Coolers

If you are building from a kit, the manufacturer no doubt recommends an oil cooler and may even provide a package that includes an oil cooler, oil lines, fittings, mounting hardware and air ducting. In that case, there’s no reason to reinvent the wheel. Just use their package and be done with it. Similarly, if you are using a salvaged FWF package from a used or wrecked certified airplane, the oil cooler used on that airplane is most likely a good choice for your plane too—just make sure it’s in good condition.

The amateur builder can use an oil cooler that has all the FAA approvals or go with a more economical alternative. Most builders do not use an FAA-approved oil cooler unless they got it used for a good price, because the cost may be two to three times higher. A word of caution about used coolers: Flushing out and repairing a used certified cooler could cost as much as buying a new non-certified one. This work should be done by a shop specializing in reconditioning and is not something most builders can do on their own.

PosiTech and AERO-Classics oil coolers are popular among homebuilders. They make a number of good products starting at less than $200 for a four-cylinder Lycoming, and they are readily available from aircraft parts suppliers. If the kit manufacturer or plans provider has not recommended a specific cooler, these companies can offer suggestions. Larger engines need larger coolers, and slower airplanes need larger coolers because they push less air though the cooler, all other things being equal. It is good to consult other builders to see what has worked well for them in terms of brand, size and mounting details.

This RV builder has mounted his oil cooler to the baffling behind #4 cylinder. Note the added reinforcing to carry the weight of the cooler.

This builder has fashioned his own shroud for his firewall-mounted oil cooler.

Getting enough air to and through your oil cooler is every bit as important as choosing the right size. There are two basic choices: Either take air from the engine baffling and divert it to the cooler, or bring in outside air through a scoop of some sort. It is common to pull air off the back of the engine baffling behind a rear cylinder and run it through a short piece of SCAT tubing to the oil cooler. This SCAT is typically 3 to 4 inches in diameter, depending on the size of the cooler. Where the SCAT meets the cooler, a shroud of some sort is needed to channel all the air into the cooler. Van’s Aircraft sells a sheet-metal shroud that is light and functional, but it does little to optimize airflow. Glasair sells a molded fiberglass shroud that is a bit larger and does more to smooth the flow of air into the cooler. You can also make your own without too much trouble.

This GlaStar builder chose to use a NACA scoop to get air for the oil cooler. This left more air available to cool the engine.

If you choose that route, visit the web sites of the companies listed above and see what their products look like. Using your oil cooler as a pattern, you can bend sheet metal into a shroud using your vise and some hand tools. A flange from any supplier will let you easily connect to the SCAT without any difficult fabrication. You can also make a fiberglass shroud by first making a plug mold out of poster board and tape or Styrofoam, then covering the mold with fiberglass. That is obviously an oversimplification, but it really isn’t difficult if you are comfortable working with fiberglass.

Whatever cooler and shroud you come up with, mounting the cooler securely is important. Sometimes you can simply use Adel clamps and secure it to the engine-mount framework. Mounting the cooler to the firewall works too, if you can secure it to some more substantial structure behind the firewall. But simply relying on thin metal to hold the weight of the cooler is not a good idea. A good practice is to use two pieces of 1 x 1 x 0.062 aluminum angle bolted through the firewall to some structure. The oil cooler can then be positioned along the length of these angles to make it fit with the other items in the engine compartment. Note that you should not drill through a structural tube without making provisions to restore the strength of the tube around the hole. When in doubt, consult the kit or plans provider.

RV-8 builder Eddie Rohwedder considers possible locations for his oil cooler.

In most cases, mounting the cooler horizontally, with the hose connections facing forward, will give the best airflow through the cooler, but each installation must be evaluated in light of everything else sharing this space. In other installations, the cooler mounts directly to the rear of the engine baffling, behind one of the rear cylinders. If the engine mount will allow this, that arrangement can be efficient. However, be sure that the baffling is strong enough to support the weight of the cooler and the oil in it, with a healthy allowance for G loads that may be placed on it during emergency maneuvering. Also, remember that the air must flow out of the cooler even more easily than it flows in, because air going through the cooler will expand when heated by the oil.

Oil Lines and Connections

The engine oil must get from the engine to the cooler and back again. This should be a simple process, with the minimum number of fittings and the shortest amount of hose that is practical. Keeping it simple saves weight and money. However, be sure to make the lines long enough to allow for engine movement.

In most installations, the oil-pressure line (the one feeding oil from the engine to the cooler) comes out of a threaded 3/8-NPT pipe opening in the accessory case at the rear of the engine. Into this hole goes an AN816-8 nipple that allows a half-inch (-8) hose to connect to the engine. An angled AN823 fitting can also be used here in some installations. Then a flexible oil line carries the oil to the cooler.

Oil coolers typically also have 3/8 pipe threads and will need AN816-8 nipples or possibly AN822-8 elbows. Threads on all of these fittings should be sealed with Sealube or another compatible thread sealer. The sealer goes only on the pipe-thread portion of the fittings, not the hose-fitting portion. Do not use Teflon tape on these or any other airplane pipefittings. Oil normally returns back to a dedicated fitting at the top of the engine near the breather fitting, where it can drain back into the crankcase without restriction.

Aeroquip 303 or 601 hoses are popular choices. The 303 hose is stronger and less expensive, but many builders prefer the look of the stainless-steel braided exterior of the 601 hose, and its 1000-psi maximum pressure is more than enough for an oil line. There are other brands, of course, but Aeroquip is among the most popular. Brace yourself when you go to buy these hoses and fittings—they’re pricey. The 303 hose fittings also require a mandrel for proper installation, but the 601 fittings do not. Both the mandrel for -8 fittings for 303 hose and the -6 for fuel lines run about $65.

Cutting Aeroquip hoses is not difficult. A small die grinder or Dremel tool with a thin, abrasive cutoff wheel works well for this task. First, tightly wrap three layers of duct tape around the hose where you wish to cut it. Then cut through the hose with the abrasive cutoff wheel. Be sure to follow the manufacturer’s instructions for installing fittings on any oil lines.

The most accurate measurements can be made with the actual fittings in place.

Wrap duct tape around the hose and transfer the cut mark to the tape. Then cut the hose with an abrasive cutoff wheel and a Dremel tool or die grinder. Be sure to blow the hose out after cutting.

Many builders are reluctant to make their own hoses, but you can save some money by doing it yourself, and it isn’t difficult. The other advantage is that it is easier to get the lengths right if you can make up one end, put it in place and then mark the exact location of the other end. Using a tape measure or wire to determine hose length can get you close, but there is always the possibility that you and the hose technician won’t have the same understanding of how you determined your length.

When installing the finished hose, be careful not to over-tighten the fitting nut. It is only thin aluminum, and it should not be attacked as though you are arm wrestling King Kong. The short aluminum wrenches that are made just for these fittings will help discourage over-torquing.

Push the end of the hose into the nut portion of the fitting until it bottoms out.

Tighten the fitting until it almost bottoms out on the nut.

Check the gap to be sure the fitting is correctly assembled. The gap should be between 0.005 and 0.031 inches.

Oil lines need to be well secured to avoid flopping around and chafing or contacting hot exhaust pipes. Many builders use nylon zip ties to secure hoses, but these have their limitations. Cushioned Adel clamps (MS21919) are much preferred for such installations, but they can be difficult to install. Aircraft Spruce & Specialty, Wicks Aircraft Supply and Avery Tools all sell special tools for installing Adel clamps that can make the work much easier, especially when installing two clamps at once to secure a hose to an engine-mount tube. Much frustration and foul language can be avoided with a small investment of $35 to $40.

Adel clamps are the preferred way to attach oil lines to engine-mount tubes or anywhere else. This special tool, available from Avery Tools and other vendors, makes installing clamps much easier.

If zip ties are used, install them with standoffs to avoid chafing. You’ll also need to carefully examine them at each yearly condition inspection to be sure there’s no chafing. The worst way to secure hoses is to wrap them around whatever tube or bracket may be handy. This virtually assures chafing damage over time.

Oil lines, fuel lines and wire bundles should be routed and secured to prevent them from coming closer than 1½ inches to any exhaust pipe or muffler. Special care should be taken to keep zip ties even farther away when possible, as heat will quickly damage them.

One last note: Engines move, especially those using Dynafocal mounts. Leave enough slack in all wires and hoses that connect to the engine to permit engine movement. When in doubt, leave a little extra slack.

Oil Filters

An oil filter should be installed on every airplane engine. Filters remove contaminants and allow for the inspection of any small particles that may be suspended in the oil. Further, they allow for the oil change interval to be extended from 25 to 50 hours, thus paying for themselves quickly. Most engines come with oil filters, but those that do not can be retrofitted with a number of available options. ECi and B&C both make oil-filter adapters that are similar to stock Lycoming oil-filter adapters, priced at just under $400. Another option is a remote filter such as the one made by Airwolf, whose remote filter kit costs almost $500.

There are two oil-filter-related tools that you should invest in. The first is an oil-filter torque wrench. Oil filters need to be properly torqued and not over-tightened. This can be done with a conventional torque wrench and a crow’s foot, but a dedicated oil-filter torque wrench works much better, especially in the tight spaces where oil filters always seem to be installed. The second tool is an oil-filter cutter, which lets you inspect the oil-filter media for small particles that can warn of engine wear and possible impending disaster. This inspection should be done at every oil change.

Crankcase Breathers

Crankcase breathers are not exactly oil lines, but this seems like as good a place as any to discuss them. Every engine needs a crankcase breather to relieve combustion gases that inevitably leak past the rings and valve stems. Without a breather, seals and gaskets would quickly blow out, making an oily mess of the entire engine compartment and leaving your engine dangerously low on oil. The breather connection is a ¾-inch slip-on hose fitting at the top of the accessory case, right next to the point of connection for the oil-return line from the cooler. The breather hose needs to vent overboard, typically near the bottom exit of the cowl. It is desirable to have an oil separator near the highest point of the breather line to catch as much oil vapor as possible and return it to the crankcase.

The M20 air/oil separator works best if it is mounted as high on the firewall as possible.

Andair makes a nice air/oil separator that drains condensed oil into a catch can instead of back into the engine.

The oil separator is simply a can with baffles inside and a ¾-inch hose inlet and outlet and a ¼-inch oil-return hose outlet. The M20 oil separator is popular with owners of certified airplanes, and it also works well in Experimentals. Low-cost alternatives are available from the usual sources. I’ll offer a few more pointers about breathers. A breather line should go uphill as it leaves the engine and continue uphill as far as possible. This allows any oil that condenses in the breather hose to drain back into the engine. The oil separator should be mounted as high as possible in the engine compartment to allow plenty of room for the drain line to gravity-feed back to the engine. And the exit to the breather should be configured so that if ice plugs it up, there is an alternative path for the crankcase gases to escape. This alternate exit from the breather tube is called a whistle slot or hole.

The outlet from the breather hose needs an alternate exit in case the main line gets plugged with ice or debris. Note the whistle hole at the end of the rubber hose.

Controlling Oil Temperature

People who live in areas with large temperature swings from summer to winter may need to make provisions to restrict the flow of air into the oil cooler to keep the oil temperature above 165° during the colder months. After sizing the cooler for summer temperatures, this can be a real problem for people living in cold winter climates. It may be necessary to block off half or more of the airflow to the cooler to keep temperatures in an acceptable range during the winter. This can be done with an adjustable butterfly valve or simply a plate that blocks the airflow to a portion of the cooler. The parts from a carburetor heat valve can often be modified to make a butterfly valve for oil-cooler air. Another option is a butterfly valve made especially for oil coolers by NonStop Aviation ( A ratcheting, cabin-mounted control can be used to adjust airflow as needed. A fixed plate can work too, but you need to remember to remove it after the winter snows start to melt.

GlaStar builder Vic Chewning used parts from a carburetor heat valve to fashion a damper to restrict airflow to his cooler during cold weather.

The oil cooler is an important part of engine cooling. It is a simple system, but that simplicity doesn’t mean it should be taken for granted. Set it up correctly, and it should be trouble-free for years, helping your engine live a long and happy life. Next time, we’ll look at fuel systems with an emphasis on FWF components.

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Dave Prizio
Dave Prizio has been plying the skies of the L.A. basin and beyond since 1973. Born into a family of builders, it was only natural that he would make his living as a contractor and spend his leisure time building airplanes. He has so far completed four—two GlaStars, a Glasair Sportsman, and a Texas Sport Cub—and is helping a friend build an RV-8. When he isn’t building something, he shares his love of aviation with others by flying Young Eagles or volunteering as an EAA Technical Counselor. He is also an A&P mechanic, Designated Airworthiness Representative (DAR), and was a member of the EAA Homebuilt Aircraft Council for six years.


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