This month’s Ask a Tech question comes from the owner of an older Bonanza who’s dealing with an unexpected engine overhaul and is starting to feel the pain of the extras that add to the invoice. “My shop is insisting on sending the oil cooler off for overhaul, but I never had cooling issues and the engine and cooler never leak a drop of oil. Why should I spend nearly $1000 on this service?” he asked.
The short answer is the same reason you wouldn’t reinstall an old oil filter on the new engine. Moreover, oil coolers—as simple as they may be—are accessories that most owners don’t really think about and don’t always get the attention they deserve on the shop floor. Most specialty shops will advise the average lifespan of an engine-mounted cooler can be around 10 to 12 years, but failure can occur much sooner. Common failure modes include stress cracks and corrosion-induced leakage. Remote coolers can live a long time, but only if serviced regularly and properly.
A tough life
A hard failure isn’t the only reason to service or replace a cooler. The other concern is blockage. By nature of its design, the oil cooler makes for an effective secondary filtration system, catching carbon, metal and other engine contaminants. We’ve seen failed attempts at resurrecting them because simply flush-cleaning a cooler probably won’t do it much good. The best shops have the cleaning process down to a science. One shop we can highly recommend is Pacific Oil Cooler Service in La Verne, California, which is also the sister company of oil cooler manufacturer Aero-Classics. That’s a company tech flushing a cooler in the image below.
After ultrasonic cleaning, it’s an involved flushing process where each cooler spends time on three different flushing machines. High volumes of Stoddard solvent flush at high pressure—with reversing direction of flow every 60 seconds—is the key to thoroughly flushing a cooler. The final flushing bench has a 10-micron filter screen that is carefully monitored until the cooler is clean. Coolers are run on each of three flushing benches over the course of a couple of hours.
Same task, varying designs
Think in terms of an engine radiator. An oil cooler has rows of air fins separated by narrow oil passageways. On the inside, the oil flows through the oil passages and is slowed by a turbulator plate, dispersing the oil while radiating heat into the air fins—subsequently radiating heat into the ambient airflow.
For our reader with an STC’d IO-550 Continental engine on his Bonanza, the oil cooler is mounted directly to the engine and that might mean more frequent servicing compared to one on a typical Lycoming engine with remote coolers. But there’s no free lunch. The advantage to the direct-mount Continental design is its lack of external oil transfer hoses (which can be a source of oil leakage), but damage usually means replacement.
We scanned the Aircraft Spruce catalog for typical replacements, where prices are all over the board. An Aero-Classics front-mount non-congealing cooler for a Continental is around $1300 while a rear-mounted model is around $2600. But many shops might offer overhauled exchange coolers. We were quoted $450 for an Aero-Classics overhaul exchange model for a Lycoming IO-360. An NDM cooler with a Stewart Warner cross-reference part number for a Lycoming was $550 for a new exchange. Talk the options over ahead of time with your shop so there aren’t surprises, but accept that an exchange or high-quality servicing is an important investment.