I’ve always thought the term “breaking in a new engine” was a strange expression. What are you breaking? Why not call it wear-in? Well, we can’t change tradition.
The main part of break-in is to wear in the cylinder walls to provide a better sealing surface for the piston rings.
When a new cylinder is ready for installation, there are honing marks on the cylinder surface. If you magnify the surface many times over, it would look like sharp peaks and valleys. When the engine starts, the valleys fill with oil splashed on the cylinder walls and the rings ride on the sharp peaks.
During break-in, the sharp peaks are broken off — this may be the origin of the term “break-in” — and the amount of oil in the valleys reduced. As the peaks are knocked off, the rings seat better due to the increased contact area for the rings.
What most people do not realize is that a small amount of oil is in these valleys and ends up above the rings and exposed to the combustion process.
I’ve heard estimates that in about five minutes of normal operation, the total volume of oil in the crankcase will be above the rings at some time.
Is seating the rings the only part of the break-in process? Not really.
For example, the cam and lifters sort of wear-in during the break-in process for a new engine. Most new or reground camshafts have the face ground at a slight angle to ensure that the lifter rotates.
However, the manufacturing process is not perfect. Many times the lifter bores are not at a perfect 90° to the cam, the cam lobe surfaces are not a perfect angle, and the lifter face may not be perfectly to spec.
Another factor is hardening of the various surfaces. When a new engine is started, all of the surfaces wear in and conform to the other components.
In the past, NASCAR race engines had to run flat tappet cams (they now allow roller lifters). The normal process for these older engines was to break-in the engine with low load valve springs. Then when the engine was entered into a race, they would change out the valve springs to much stronger ones. In this way, the cam and lifters would wear in and make up for any manufacturing imperfections.
The other part of this is that the break-in process actually hardens the various parts.
There is also an alloy process, especially with oils containing zinc. I know that aviation oils do not contain zinc, but many people use the additive STP and it does contain a zinc additive.
A third area in the engine that is affected by the break-in process is the valves, particularly the exhaust valves.
I have told the story about when we were running valve recession tests back in the late 1960s. In these tests, we ran the engines that had been run for a very short time on leaded fuels for several hundred hours with no exhaust valve recession. Then we re-ran the exact same test procedure with new heads and valves. In these tests, there was a significant amount of exhaust valve recession.
The same is true with aircraft engines. When aircraft engines are started on leaded fuels and then switched over to unleaded fuel, there is usually a very low number of cases of exhaust valve recession reported compared to cases where the same type engine is started from new on unleaded fuel.
I am not absolutely sure of the mechanism here. I have heard some people say that the lead coats the exhaust valve seat and provides a cushion. Others claim that there is an alloy or hardening process for the valve and seat.
So what is the best way to break-in a new or overhauled engine? Should you operate your new engine hard or baby it? Should you use mineral oil or start from new with AD oil?
The best advice here is to follow the recommendation of the people who overhauled or manufactured your new engine. This is especially true for your warranty.
That’s interesting that when an engine starts with leaded fuels and then switches to unleaded fuel, there is usually a low number of cases of exhaust valve recession. If I owned a plane, I would be sure to get the right aircraft engine overhaul. I think that taking care of the valves by using the right fuels is also important to keep good maintenance for an aircraft.
Thanks for helping me learn more about breaking in an engine. I actually didn’t know the surfaces will be worn in to conform with other parts. I’m kind of interested to visually see what this is like or how long this process could take.
Hello, couldn’t resist in putting in my two cents worth. I worked for an aircraft engine rebuilding Co. and have flown several aircraft with overhauled engines to seat the rings on to the cylinders. We would climb to altitude slowly as not to heat the engine above normal temps. After that, we would put the pedal to the metal and fly the aircraft back and forth at altitude above the airport till we could hear the engine change pitch, and the noise level would come down considerably, usually between 10 and 15 hours of running the engine, most of the time hard, we would on occasion change the throttle up and down to make sure we were getting top ring lube and not getting hot spots on the cylinders sort of speak. Otherwise, the engine would run pretty much flat out at full throttle. When you hear the engine change pitch and become quieter it was pretty obvious the rings were starting to seat, you could hear the cylinders one by one start to come in until they were all quiet and seated. A few hours after that we would dump the mineral oil and go with the AD type aircraft oil. End of story, never had a failure or recall, works well, what my boss would tell us, ” don’t baby the engine, fly it hard so as to seat the rings, if you baby it you run the chance of glazing the cylinders and that’s bad.
Very interesting indeed.
What about leaning procedure when breaking in the engine? Any difference from normal?
And how do you recommend the leaning should be?
I too, read this looking for an article on breaking in a new engine. So, your advice boils down to- do what the manufacturer/overhauler tells you too do. Umm, thanks.
Got the following explanation from https://oldschool.co.nz/index.php?/topic/20751-valve-seat-recession-explained/
Valve seat recession (VSR) is the name given to a process of accelerated valve seat wear. The mechanism by which this occurs is as follows:
1. The very high temperatures at which it operates (typically between 600 and 800 degrees Celsius) causes the exhaust valve to weld locally to its seat. When the valve is then opened a small particle of the iron seat is pulled away. This particle is converted into iron oxide in the hot exhaust gas.
2. As the valve then closes again these particles are imbedded in the valve face.
3. With these hard lumps of iron oxide imbedded in it, the exhaust valve becomes, in effect, a grinding wheel. Thus any rotation of the valve will cause it to grind away the seat. Valves tend to rotate under the action of the valve springs and by engine vibration.
The ‘seat’ and ‘valve’ protection effects were something noticed later on, after WWII [i think]. Tetra ethyl lead leaves a microscopic layer of lead on the valve and seat; some folks call this cushioning. It was generally originally accepted that this effect, a ‘lubricating’, if you will, prevents microscopic welding of seat and valve, or at least reduced metal to metal microscopic contact, sort of how oil works.
Lead is, at very high temperatures, a LUBRICANT, AND A TERRIFIC CONDUCTOR OF HEAT. The conjecture was that a small amount of lead makes the seat and valve much less likely to weld at microscopic scattered points, and that the valve head heat is more readily transferred to the seat. Lead was thought to generally totally prevent the welding.
You write an article titled breaking in a new engine and then offer no discussion of breaking in a new engine.
Exactly, what is the purpose of this article?
Continental break-in specifications , on our engine, were to run it between 75% and 65% power, full rich, at low altitude. We had to watch the CHTs to keep them below 420 degF when at 75% power. After about 2 hours the CHTs settled down to 400 deg. The oil consumption came down at about 15 hours.
On take off, we had to do shallow climbs at 100 mph to keep the CHTs down and level off a couple of times to cool the cylinders.
What is “exhaust valve recession”?
With reduced TEL in avgas, the exhaust valves tend to run hotter . We had one old cylinder, with about 900 hours on it, wear the exhaust valve seat which caused the exhaust valve to move into the head. This resulted in the valve stem pushing the rocker arm up to the point where the rocker arm contacted the valve spring retainer.!
This cylinder went to ‘0 ‘ compression with extreme wear to the exhaust valve guide.
Exhaust valve guide wear is a common problem on Continental cylinders, but this is the first time I’ve seen the valve seat wear this badly.