This article originally appeared in the February 2001 issue of Light Plane Maintenance, and is reprinted here by permission.
The day has come. It’s finally time to overhaul that engine. Or maybe it’s just a top overhaul. Either way, it’s a not-so-often event and there’s slight fear and a little anxiety.There is no shortage of horror stories of jugs coming off six weeks later for re-honing and new rings or sometimes-new pistons. It certainly is enough to give most owners the willies. With all the different methods of breaking in an engine, it’s no wonder that the worst occasionally happens.But it really shouldn’t. There’s nothing mysterious about break-in procedures and there’s probably more than one way to do it correctly. When you’re shopping for an overhaul facility, just ask around and you’ll see what we mean. Everyone has a procedure. Trouble is, they all tend to be different. Some a little and some a lot.To try and clear things up a bit, we’ll take years of experience and condense it into useful advice that will keep you from trashing that new engine or top overhaul. The trick is to avoid fatal mistakes, most of which involve overheating the cylinders.So what’s the best way to break in your particular cylinders? What happens during break-in anyway? To state it plainly, the main objective of break-in is to wear the rings to the cylinders and get the valve and seat sealing surfaced to mate accurately to each other.
The cylinder barrel is the only point on the engine where metal-to-metal contact is desired at any time. However, it’s only desirable during break-in and not during the rest of the engine’s life.This metal-to-metal contact is necessary to “match wear” the rings to the cylinder walls, something that’s required for good compression and long top-end life.The cam, lifters and other bearings wear into each other, too, but it would be just as well and very advantageous if there was no metal-to-metal contact there at all. But the ring seating process is accomplished by actual metal-to-metal contact between the ring and the cylinder wall.The ring’s job is to seal all the gases it can on the top side of the piston from escaping to the bottom side of the piston and into the crankcase. To accomplish this, the ring must match the contour of the cylinder barrel along the ring’s full diameter and width.The cylinder barrel must also do some wearing to have a consistent contour along the stroke of the ring. This will enable heat to dissipate quickly through the rings, keeping the rings and piston cool.This also makes for a tightly sealed combustion chamber, which makes for a healthy, efficient and powerful engine and an acceptable level of oil consumption that’s a trade-off between wear reduction and good compression sealing.
Getting it Done
One of the big factors in the break-in is the hone job done by the manufacturer/cylinder shop. If it’s too rough, it will wear rings too quickly and increase oil consumption. Too fine a finish will stop the process prematurely and leave you with low compression and mediocre oil consumption.This is out of your control, however, so we’ll talk mostly about the thing you can control: the run in. The key to this process is to do it reasonably quickly and with moderately high power. If it’s done too quickly the cylinder walls will gall and ruin the rings; too slowly and the rings will anneal and glaze the cylinder walls.Either one of these problems will require removal of the cylinders and a cylinder re-hone, and installation of new rings — possibly even new pistons. The secret to break-in, if it can be called that, is this: allowing the rings to wear on the cylinder walls without getting them too hot.Some pilots don’t understand the importance of the ring temperature factor and will glaze the cylinders every time. The rings are made of tempered steel that’s affected by temperature. The higher the temperature of the ring, the less time required to anneal or remove the “spring” from the metal.When the ring anneals, it loses its ability to exert pressure against the cylinder wall. This inhibits the transfer of heat to the cylinder, causing the ring to retain heat and anneal further, thereby aggravating the situation.
An easy way to tell if an engine has been overheated is to attempt to twist the top compression ring. (Obviously, this assumes the engine has been disassembled, or at least a cylinder has been removed.)A new piston ring, or one in good shape, can be bent up to about 70 to 80 degrees before it breaks. An annealed ring will bend past the 90-degree point and a bad one can even go all the way around on itself, a full twist.When the ring is in this condition, it loses its heat transfer ability; the heat builds up and overheats the oil around it, causing the brown varnish deposits you see on a glazed cylinder wall.At this point, the ring does a poor job of sealing, causing low compressions by the rings. Blowby also heats and burns the oil and adds to the small fiscal catastrophe.So it would seem that a good thing to do is to keep the piston and rings cool.That’s exactly the case. When you’re running in a new engine, do your best to run 100 to 150 degrees rich of peak EGT or TIT, whichever way your aircraft is equipped. If your aircraft isn’t equipped with either, richen the mixture as usual and then richen the same amount again.Example: On most carbureted engines without EGT, you richen until rough or to power loss, whichever comes first. Then richen to smooth or best RPM. For break-in, add the same amount of richening again.This will give you one to two gallons per hour more fuel flow. That’s what’s required to keep the pistons and rings cool in a carbureted engine.In a fuel-injected engine, you’ll have much more control over leaning and the resultant temperatures. In some fuel-injected engines, it may be possible to run lean of peak EGT, since this will actually result in cooler temperatures than some rich-of-peak settings.However, if the engine run won’t smoothly lean of peak, stay on the over-rich side. Also, in a normally aspirated engine, lean-of-peak operation may not produce cylinder pressures high enough to force the rings against the cylinder walls, something that’s also a critical part of engine break-in.
In most situations, break-in will be done within 50 or fewer flight hours. The majority of the ring seating happens in the first five to 10 hours and the seating of the full width in another 40 to 50 hours. Some experts believe that fully 90 percent of break-in occurs in the first hour, so the first few hours of engine operation are critical.There are as many ways to do this as there are engine shops. Some shops start the break-in process in a test cell, with the engine wired up with instruments.
This is only the beginning, however, since that run is rarely more than an hour or two. Some shops actually tow the airplane out to the run-up area at the airport, start it up and immediately take off, flying the first several hours at low altitude and high power.Again, this gives the owner a leg up on break-in, but it’s only a start.For all engines, determining break-in is a combination of oil consumption and compression test readings. Once the oil consumption has stabilized and the compressions are in the high 70s, it’s safe to go back to the detergent oil and fly at a more conservative mixture setting. For example, 75 degrees rich of peak or so (stay with what your POH recommends), or if you believe in lean-of-peak, 25 or 50 on the lean side.An engine is considered fully broken in somewhere between the 50- and 100-hour mark. But normal operation can be resumed around 10 hours after oil consumption has stabilized.Do your utmost to keep pressure on the cylinders (engine driving the prop, not the other way around) and run at 70 percent power or perhaps 75 percent on the lower-horsepower engines. As usual, long cross-countries are the best medicine for break-in. The cylinders get consistent ring pressure and the process happens much more quickly.Touch and goes and low-power operations can prevent and even ruin the break-in. Don’t do them. Also, avoid long, power-off descents. Whether you believe in shock cooling or not, these aren’t good because they reduce the ring-to-cylinder pressure, thus slowing the wear-in process.Some say that varying the RPM every so often will help break-in. We haven’t seen anything that would empirically support that idea but as long as there’s pressure on the cylinders, it shouldn’t hurt, and will probably help avoid spark plug fouling, if there is any tendency toward this.What we actually do see is that the high-RPM engines — the geared models — rarely have a problem in break-in, even if they might be nightmares to operate later on. We have yet to hear of a Duke, Cessna 421, P-Navajo or Helio-Courier coming back into the overhaul shop for glazed cylinders because the rings just wouldn’t break in.Even with the detergent and multigrade oils, these engines seem to always break in easily and are usually fully broken in at 40 or fewer hours. (Good thing; these are among the most expensive engines to overhaul.)
Turbocharged and high-compression engines can be more sensitive to oil types on break-in. Lycoming states in SI1014M that you’re never to touch one of their turbocharged engines with a straight mineral oil. From the experiences gleaned from overhaul shops, this has validity.Turbocharged engines run much hotter than naturally aspirated engines, especially as the pistons and rings are concerned.With the constant and usually higher pressure on the rings in a turbocharged engine, the rings will have enough contact with the cylinder walls to break in quickly and fully in 25 to 50 hours.It’s still advisable, however, to operate the engine 100 to 150 degrees rich of peak, especially in some of the tightly cowled, high-power engines that Lycoming produces. Some good examples of that are the Piper Turbo Saratoga/Lance and the TIO-540 Navajos.These engines run hot routinely so it’s in the owner’s interest to keep them as cool as possible during the break-in period. Don’t exceed 400 degrees F cylinder head temps or you’ll likely anneal the rings. The rings on these engines usually anneal to some extent anyway so don’t give them any reason to get worse.It’s also worth noting that the turbocharged engine can be run at 65 percent power and still enjoy a quick and gentle break-in. The positive cylinder pressure on every stroke helps the rings stay in contact with the cylinder walls without having to push the engine to higher power settings to achieve same result.That’s why a naturally aspirated engine needs 70 to 75 percent power while a turbocharged motor usually requires only 65 percent power. For turbo-normalized engines, run 70 percent as long as the cylinder head temps are below 400 degrees F.Obviously, in a turbocharged engine, altitude may not be much of an issue, since you can maintain high power output at relatively high altitudes. The same is true of a turbo-normalized engine.However, high-altitude cruise has one disadvantage: Cooling is less efficient in the thin air of the upper altitudes. So even though you’re able to achieve 70 percent power well into the teens, the CHTs may get too high.Until the engine is broken in, pick a happy medium determined solely by cylinder temperatures. Anything below 400 degrees is acceptable; cooler is better.Normally aspirated engines will have to be flown at low altitudes to achieve the desired power output. In most cases, that’s going to be below 8000 feet and probably below 5000 feet in some cases. Again, the higher the power output, the better, consistent with keeping the CHTs in line.
Conventional chrome cylinders differ in break-in in that the rings do probably 95 percent of the wearing to break-in and seal the combustion chamber. This takes about twice as long compared to steel cylinders and will always yield oil consumption higher than one quart per 10 hours. Perhaps quite a bit higher.That’s just the way they go. It’s been that way for 50 years so you might as well get used to it. Chrome cylinders are known for low wear rates and better corrosion resistance against the trade-offs of using a little more oil.ECI clearly acknowledges in their break-in manual that chrome is the most sensitive to proper break-in than any other type. In fact, ECI feels that with the exception of chrome, regular Phillips X-country oil is perfectly fine from the first hour on — as far as using their cylinders is concerned. The channel chrome cylinder finishing process has become far less popular. (See comments on CermiNil below.)There were attempts to improve on channel chrome’s service record, most notably with products such as CermiCrome. However, The success of these variants has been uneven, at best, and has essentially passed into aviation engine history.
ECI’s CermiNil process has been around for several years now with tens of thousands of cylinders in service. This process seems to be doing significantly better than any predecessor treatments with no downside we are aware of. It’s more than a different material; it’s a different process. We have heard nothing but praise from owners.For folks who fly fewer than 100 hours a year or with long intervals between flights, CermiNil is probably the best way to go because of its greater corrosion resistance and easy break-in.
With all the attention focused on rings and barrels, the tendency is to forget the all-important part that the break-in process plays on the valves.When the engine is assembled, the valves are lapped and seal well, or at least they should. This will change, however, when those valves are exposed to the heat of combustion and the rapid open/close cycle of routine operation.It’s important to seal the exhaust valve face so it will seat quickly during break-in, to avoid burning a valve in the first few hours of flight. All the lead in 100LL and the extra-rich mixture for the piston/ring temp consideration does wonders to seal and slightly groove the face of the exhaust valve to its seat.Once this happens, the valve won’t give you any problems unless you overheat it with too lean of a mixture at higher power settings. This brings up the inevitable question: What do the folks who run their engines on an auto gas STC do? The answer is to burn 100LL for the first 40 or 50 hours.After that, we recommend keeping a 10- or 20-percent avgas-to-auto gas ratio in the tanks to keep the exhaust valves in good shape. Many an owner/operator has burned up two out of four or four out of six exhaust valves by not running any avgas in the fuel. If you have had no issues with straight autofuel, fine; we are only passing on our conservative position. The lead is critical to exhaust valve face/seat lubrication and sealing — so put some avgas in for your engine’s and wallet’s sake. If you have operated on auto gas without ever touching 100LL after break-in, congratulations; you are beating the odds.We might note that Petersen Aviation, who holds and sells many auto gas STCs, recommends a 25-percent mixture of avgas for Franklin engines and radials. They also recommend that you burn a tank of 100LL every 75 hours — even with their auto gas STC — in order to help out the valves. Need we say more?
Cam and Lifters
Although we would like to never see cam and lifter contact, it’s inevitable because of the extreme pressures put on the lobe and lifter. The name of the game here is to run the engine frequently until it’s broken in.Each lifter and lobe wears into the corresponding surface in a particular pattern. From the engines we’ve seen and the owners we’ve talked to, the cam seems to be especially vulnerable to corrosion until it’s worn in to its lifter.The worst thing you can do to a new or newly overhauled engine is to fly it once or twice and then let it sit. The heat of lots of flying and the break-in process help the oil and some desirable anticorrosive deposits to bake into the internal components of the engine, giving some protection from corrosion on a short-term basis.The airplane should always be flown at least once a week. That should be standard practice for good engine health through the TBO run in any case. Some long cross-countries will do wonders in the beginning that will pay off as you sail — hopefully — well beyond the 1000-hour mark.So, fly it often. Fly it long. Fly it cool and have at least some lead in the fuel. Oh, and one last thing: Change the oil more often rather than less often. If you can’t fly the airplane as much as you’d like, that will at least help keep corrosion at bay.
A Final Word
We have presented what we feel is an effective, time-proven approach to break-in. That said, we strongly recommend that you discuss this with whoever does your overhaul. The last thing you need is someone denying warranty coverage because you did not follow their specific directions.This is especially important with factory engines. The break-in procedures are outlined in their Service Bulletins, and they expect that you will follow them to the letter if you expect warranty coverage.