By Lisa Turner, EAA Lifetime 509911
This piece originally ran in Lisa’s Airworthy column in the December 2020 issue of EAA Sport Aviation magazine.
“Scotty, I see you have your Cub engine back from the overhaul shop. How’s it running?”
“So far, so good, Eddie. I’ve been coddling it, treating it gently, and running it a lot on the ground to get it broken in before I fly it.”
Eddie scowled at Scotty and shook his head from side to side. “Uh-oh. How long have you been doing that?”
“What do you mean, ‘Uh-oh’? I’ve been doing it now for three days. I have six hours of ground running and taxiing. I would think with a new engine overhaul, you’d want to take good care of it at first, right? The year is 2020, I can’t imagine you have to do any break-in nowadays.”
Eddie closed his eyes and waited. Scotty looked at him with concern.
“What kind of oil temperatures are you seeing when you’re doing this?” Eddie asked.
“A lot hotter than normal. They will come down after break-in right?”
“You hope,” Eddie replied. “Didn’t the overhaul shop give you instructions for break-in?”
“No, they said just go fly.”
“Well, they said go fly. You shouldn’t be ground running it for hours,” Eddie said. He struggled to explain why break-in was still important in aircraft engines.
“I don’t understand,” Scotty said.
“Okay, let’s go into the hangar and sit down with a cup of coffee and a pad of paper, and I’ll explain it.”
But the damage had been done. Scotty took the airplane, now with 25 hours on the newly overhauled engine, to the mechanic on the field.
“It’s never been right,” Scotty said. “It runs hot, it burns through oil, it’s using a lot of fuel, and the power just doesn’t seem right.”
After an inspection, the mechanic turned to Scotty.
“You’ll need to take this back to the overhaul shop,” the mechanic said. “They’ll need to rehone the cylinders and install new rings. And then you and I will talk about engine break-in and how to do it right.”
It was Scotty’s turn to scowl and shake his head.
Aircraft Engine Break-In Myth No. 1: New engines and overhauled engines do not need to be broken in.
You may be surprised, along with me, that I have heard this from many A&P mechanics. Usually, I will say, “I don’t mean car engines; I mean aircraft engines.”
Most of the time I hear back, “Right. Car engines don’t need a break-in procedure either.”
This is a myth. Piston engines need a break-in to live a full and productive life after they have been manufactured at the factory, reconditioned, or top overhauled. Two-stroke, diesel, and turbocharged engines also have specific procedures that I won’t cover here.
Many of us will experience the joyful day when we finish our aircraft kit, our restoration project, or our engine overhaul or upgrade and be faced with a break-in procedure.
What exactly is a “break-in”? We also call it settle-in, run-in, first run, etc. According to Merriam-Webster, break-in as a noun is “an initial period of operation during which working parts begin to function efficiently.” As a transitive verb, it means: “to overcome the stiffness or newness of” a thing.
Cylinders and pistons, the heart of your engine, are purposefully mated together so that it is not a tight seal as you might think there should be. For the oil to lubricate the cylinder walls, the cylinder/piston/ring combination has to have a little space to operate in. The new cylinder wall surfaces are left deliberately with a hatched pattern so that the rings can do their final seating during the initial hours of operation. They literally scrape the metal down for a good fit.
Once this process is complete, we have just the right combination of sealing so that heat can dissipate but the rings can keep gases on the top where they are supposed to be for power and efficiency. It’s a delicate balance between wear reduction and retaining power without blowing through too much oil.
There are always exceptions to rules, and there are engines that are considered “already broken in” at the factory. This does not mean there aren’t procedures to follow for the first hour, five hours, and 10-50 hours.
The material that the components are made from — nickel carbide cylinders, for example — and ceramic impregnated materials can change how break-in is done, shortening the time for ring seating. Some materials lengthen break-in. This is the reason we should follow manufacturer recommendations precisely.
Aircraft Engine Break-In Myth No. 2: For proper break-in, engines should be babied with gentle operation for the first 10 hours.
This is a myth. Even automotive engines benefit from a break-in procedure even though the dealer says just drive it off. Nearly all automotive owners’ manuals tell you not to go too fast or drive at a constant speed in the first 600 miles. They do tell you to coddle the engine — and this advice is partially wrong. But they figure telling you to take it easy will do less damage than running full bore.
A break-in procedure for an aircraft will demand minimal ground running and high-power flying with enough time in the air to get temperatures all the way up, with high manifold pressures.
Mike Busch from Savvy Aviation wrote: “Break in the engine by running it as close to maximum continuous power as possible without allowing any CHT (cylinder head temperature) to exceed 420 degrees Fahrenheit for Continental cylinders or 440 degrees Fahrenheit for Lycoming cylinders. Run it this hard for an hour or two, until you see the CHT come down noticeably, indicating that the break-in is mostly complete.”
Aircraft Engine Break-In Myth No. 3: Use the latest, slipperiest, most high-tech oil you can find to put in your engine from the start.
This is a myth. It goes against the physics of how rings seat in the cylinder. For the best match between the cylinder wall and rings, some material must be scuffed off. If the oil you are using is too slippery, the rings can’t remove material and seat.
To do this, a nondetergent mineral oil that contains no synthetics or anti-wear additives is recommended because it’s able to lubricate enough but not too much. There may be other nonsynthetic oils and ashless dispersant oils that work, too, as long as it’s on the recommended list from the engine manufacturer. I can’t emphasize enough how important it is to use the exact oil specified for exactly the number of hours specified.
Once again, there are exceptions. Rotax, for example, specifies that the oil for regular operation should be the same for break-in. The oil you use depends on what fuel you are using in the engine (autogas or 100LL).
Break-In: What to Do and How to Do It
You’ve heard me say this before: The engine manufacturer’s advice is the advice you should use when breaking in your engine. These instructions are precise and based on thousands of hours of testing. Even if your buddy in the next hangar thinks you should use XYZ Slippery Oil for break-in, take the manufacturer’s instructions instead of your buddy’s advice. You can always try the XYZ Slippery Oil later.
Engine manufacturer advice on procedure is very specific. For example, Rotax four-stroke engines are run at the factory, plugged, and sent to buyers. When you ask them about break-in, they will tell you to pre-oil carefully using the recommended oil and pay close attention to starting procedure and temperatures. Once the oil temps are up, go fly and don’t over-rev the engine. Follow what’s in the manual.
When I hear people say, “There’s no break-in on Rotax four-strokes. They are ready to go,” I get concerned. There is definitely a process to follow to make sure you get the most life and efficiency out of the engine, even if it requires less critical care than a Lycoming.
Best Break-In Practices
You may not be able to do all of these, but doing most of them will extend the life of your engine and give you more reliable service in the form of lower oil consumption and higher fuel efficiency. With traditional aircraft engines only giving us 30 percent overall efficiency to begin with, we need everything we can get.
- Follow the overhauler’s advice/factory advice for your specific engine.
- Have your engine monitor calibrated — and set limits before you start flying. Know what temperatures you should be seeing and for how many hours. Know how much oil consumption is normal for your engine in the first five to 15 hours (when most of the break-in occurs) and for the period up to 50 hours.
- Always check for oil pressure on engine start. It’s critical at the beginning and just as important later on.
- Follow the instructions for pre-oiling from the manufacturer or overhauler.
- Follow the manufacturer recommendations for how many ground runs to perform. Although we say to limit ground runs, the manufacturer will have a specific procedure to follow. For example, the first run may be less than a minute, running at a low (1000 rpm) power setting and checking for oil pressure in the first 15-30 seconds. The second session is a little longer with more checks.
- Don’t let the aircraft sit too long in between break-in sessions. When you do fly, make it for an hour or two (after the prescribed ground checks and first flight, which typically lasts 30 minutes).
- After each session, spend time looking for problems. If they are going to pop up, this is the time frame. Look for oil leaks, fuel leaks, and component attachments, etc.
- Avoid power-off descents during the break-in period. This will reduce the engine temperatures at a time when you actually want them high.
- Follow oil change recommendations exactly. In the first few hours, the seating process will produce metal particles — a normal byproduct but one you want to remove as soon as break-in is done. I’ve heard some mechanics say not to worry about it. I do worry about it — and you should too if you want the maximum out of your engine.
When do you know break-in is done — and done right? If you’ve been observing oil consumption and CHTs, you’ll see both come down. CHTs may come down a lot, depending on the engine you have. Find out ahead of time what you should expect. It could take an hour or more than 15 hours. This is why following the factory or the overhauler’s advice is important. In addition to the cylinder/ring seal, break-in allows all the engine components to begin working in concert.
The bottom line is that you should baby your engine by following the instructions exactly for putting it in service for the first time or for putting it back in service. I guarantee that doing this will give you maximum engine life, reliability, and performance. And, after you’ve done this, listen to that special music that your engine will play for you every time you turn the key.
Lisa Turner, EAA Lifetime 509911, is a manufacturing engineer, A&P, EAA technical counselor and flight advisor, and former DAR. She built and flew a Pulsar XP and Kolb Mark III and is researching her next homebuilt project. Lisa’s third book, Dream Take Flight, details her Pulsar flying adventures and life lessons. Write Lisa at Lisa@DreamTakeFlight.com and learn more at DreamTakeFlight.com.