You just got your plane back from overhaul and have been told to “break-in” the engine for the first 50 hours or so.
What exactly does that mean and how do you go about doing that?
What happens during break-in and — perhaps, more importantly — what can go wrong?
During an overhaul, the surface of the cylinder is honed to give a cross hatch pattern. If you cut the cylinder in half and examine the cross section of the cylinder wall under a microscope, you would see the surface has tiny peaks and valleys from the honing process.
During the break-in, the rings will break off the peaks to give a slightly smoother surface to run on.
So why do you need the peaks and valleys at all? Why not just machine a very smooth surface to start with?
You need the rough surface for oil to stick to. In a good running engine, the oil in the little valleys helps to seal the rings during operation.
Most people do not realize that a small amount of oil is in the cylinder wall valleys above the rings and subject to the combustion process on every revolution. It is then washed away and replaced on the next cycle.
When an engine is first started after overhaul, the rings ride on the top of the peaks. The oil rings, in particular, are very inefficient and allow excessive amounts of oil to remain on the cylinder wall. This results in the compression rings kind of hydroplaning, resulting in ineffective compression sealing.
The result is hot combustion gasses going by the rings and raising the surface temperature. This can coke or boil off the light ends of the oil and leave a heavy varnish-like substance on the cylinder wall.
If this continues for very long, the valleys in the honed cylinder wall can become filled, which is called glazed. This super slick surface stops the break-in process and, with no oil in the valleys, the oil and compression rings cannot function properly.
You also have improper heat transfer from the piston and rings to the cylinder walls.
The Best Procedure for Break-In
The best procedure for breaking in a new cylinder or cylinders is to follow the overhaul, engine manufacturers, or cylinder manufacturers recommendation.
And there are differences.
For example, Lycoming recommends breaking in new cylinders on turbo-charged engines with ashless dispersant (AD) oil because the cylinder pressures on these models are much higher.
If you don’t have any recommendations to follow, I recommend the following steps.
After assembly, run the engine up on straight mineral oil and do the normal checks and look for leaks and proper baffle placement, etc.
Once everything looks normal, take a flight around the patch, monitoring temperatures and other parameters. If everything is in range, land and check again for leaks, etc.
If everything is good, change the oil and filter with straight mineral oil.
There’s no need to cut the filter apart and look for metal, because there will be some.
If your new engine is from the factory or a rebuilder with a dyno, you can skip that part and go on to the second oil change.
Now start using your aircraft in a normal way, but do not baby it. Low engine loads can result in inadequate ring pressure for proper break-in. Remember, we need to have enough pressure on the rings to “break off” those little peaks.
Run this oil change maybe 15 to 20 hours or so and change the oil again using mineral oil.
Now continue to fly another 25 hours or so while monitoring your oil consumption and engine temperatures. Then change the oil and look at the filter or screens.
You have now flown about 50 hours. If your oil consumption is down to an acceptable level, the temperatures are normal, and your engine is not making metal, I would consider your engine broken in and you should switch to AD oil and enjoy your plane.
If you are wondering why we break-in some engines on mineral oil and not AD oil, it is because mineral oil does not have the cleansing characteristics of an AD oil and will not wash out the metal fillings from the ring belt area. These fillings act as sort of a lapping compound to aid in the break-in process.