Product Focus: Don’t Know, Don’t Care: Oil Coolers

Oil coolers, by nature, are rather fragile. Stress on the fittings, hoses and attachments transfer to the cooler. Proper cleaning, maintenance and inspection are important to maintain the integrity of the component. Writer Tim Kern talked to experts in the field to discover how to keep your aircraft flying clean and cool.

Oil coolers are simple, trouble-free devices that require minimal care. Generally containing no moving parts and sitting inconspicuously in air ducts, they also get minimal care. Of course, on an aircraft, that’s trouble. An oil cooler failure can be benign or catastrophic, depending on your luck. How’s your luck been lately?

Fundamentally, an oil cooler should never wear out. Its insides are constantly bathed in oil, and its outsides face only air. Well, not only air; the outside is exposed to bugs, salt spray, humidity, runway runoff, birds, bees, and occasional squirrels, as well as parts of other things that get too close to the prop. Coolers are attacked by corrosion from outside, pressure on the inside and vibration. They are attached by seldom-seen brackets, served by slimy hoses, and they’re plumbed with over-tightened fittings, wrong fittings, or dry fittings that weld themselves together. Permanent and trouble-free, as I said.

Drawing on long years in car racing and depending on experts, notably Wayne Thomas at Pacific Oil Cooler Service (LaVerne, Calif., who provided the excellent photos) and Dean Vogel at Lockwood Aviation (Sebring, Fla.), I’ve prepared a short list of things to watch.

On many experimentals, many oil coolers are mounted backwards or wrong. If a cooler has a vernatherm (thermostat) in the circuit or built-on, it will be clearly marked "in" and "out." It’s essential not to hook that up backwards, for obvious reasons. More-commonly, though, there are no markings. The key is to be sure that any air that enters the oil cooler will be forced out by the oil, so be sure that the outlet is at the top. (This holds whether the cooler is mounted horizontally or vertically.)

Coolers are, by nature and construction, rather fragile. Mounts should hold the cooler without stressing it, twisting it, or transferring shock. Essential to good installation is enough slack in the hoses, and support for the hoses sufficient to keep the hoses where they belong while keeping stresses from building up at the fittings at either end.

While on the subject of hoses and attachments, be sure routing is stress-free, that the screw joints are well-lubricated (anti-seize was made for this) and that the fittings truly fit before you tighten them. Speaking of tightening, oil coolers (except some old Hanson items) have bosses for a back-up wrench, to keep torque generated when you tighten a fitting from transferring to the cooler. You don’t need to use those bosses or support the fitting, if you plan to break your cooler. (Otherwise, use them.)

The fittings that we use at various junctions are usually available in steel or aluminum. Lots of people use aluminum because they’re lighter, but experienced mechanics use steel because you can get them apart later. It’s easy to "weld" two aluminum surfaces together, and rare to do the same with an aluminum/steel combination.

If you have a remote oil tank that you clean routinely (as you should), proper cleaning and hose etiquette is important. The worst thing you can use to clean these parts is a parts-cleaning tank! It’s full of tiny particles of exactly what you do not want to put into your oil system! While the tank may be cleaned in a parts tank if you have total access to the inside and can make sure it’s surgically clean before reinstallation, hoses should be flushed only when necessary, and then only in such a way that they will see only pure, fresh solvent. As for the oil cooler itself, you flat out can’t clean one properly without special equipment. Send it to a qualified shop. Period.

Any time your engine starts for the day, the oil pressure can rise to dangerous levels if its viscosity is too high, and that happens when its temperature is too low. As a general rule, if you need an engine preheater, you need an oil cooler heater. Running an engine at elevated power settings when there is cold oil in it will build huge pressures, often resulting in a blown-up oil filter or cooler, or what Thomas calls a "football-shaped" cooler. Sometimes, the failure will be on an inside row. You’ll notice this problem when you can’t see light through your cooler. Either such condition cannot be repaired.

Even with a vernatherm, merely running a cold engine until the oil temperature comes off the peg won’t work as planned, Thomas warns. Though the cold oil will have enough pressure to bypass the thermostat’s internal spring for a moment, little oil will pass before the spring wins the tug-o-war. The result is a high-frequency internal hammering of the vernatherm and cooler, and only a tiny squirt of oil is going through. Soon, your thermostat (and most likely your cooler) will be junk.

So, make sure it’s oriented properly and keep the cooler clean on the outside (a little corrosion preventative won’t hurt, unless you let it accumulate too much dust and dirt). Use anti-seize on all your fittings (and replace the aluminum ones with steel at your next opportunity). Use back-up wrenches and proper torque when you tighten your perfectly-aligned hose ends, monitor the hose supports and routing, and pre-heat your oil cooler whenever you pre-heat your engine (generally when you’re more than 10°F below freezing). Do this and keep critters out of the intake duct, and you’ll have few troubles with your cooler.

Resources:

Pacific Oil Cooler Service, Inc.: www.oilcoolers.com

Aerotechnical Institute: www.aerotechnicalinstitute.com