Cessna 172 Skyhawk
By Ben Visser
The first decision is what oil to use for break-in.
Here, again, you need to refer to the manufacturer’s recommendations as a starting point. Lycoming recommends breaking in all naturally aspirated engines on straight mineral oil and all turbocharged engines on AD oil. (There are special requirements for the O-320H model engines.)
The difference is in the cylinder pressure required to produce adequate ring force against the freshly honed cylinder wall to wear in the surface. The biggest concern about break-in of the cylinder is glazing. This was more common with the old hard chrome cylinders when pilots would “baby” their new engines so as to not hurt them. The lack of adequate ring pressure and friction resulted in the rings not knocking off the little peaks left from honing, leaving more oil in the valleys, which can coke up, leaving a varnish or glazed surface.
With a turbocharged engine there should be adequate cylinder pressure to seat the rings even if someone tries to "baby" their engine. So always check with the manufacturer and rebuilder for their recommendations. It will make life much easier in case of a problem.
The second step is operation. If your new engine is from the factory or a large rebuild company that has a dynamometer, then the first step of the break-in has been completed.
When new pistons and rings are installed, the coefficient of frictions is very high until the smoothing out of the cylinder wall is done. Much of this can be done on the dyno run.
If this is a field overhaul, then the first few hours of the break-in will be done in the air. After these first few hours, change the oil and filter to get rid of all of the iron filings. This will have been done at the factory for a new engine.
Run this second change of oil for 10 to 15 hours and monitor the oil temperature and the engine performance. Then change the oil and filter and run this change for about 25 hours or so. If after this third oil change, the oil temperature is down to normal range and the oil consumption has returned to normal, I would consider the engine to be broken in.
A final point is fuel. All of the test results and the experience of many people in the field indicate that it is very important to do the break-in period on a fuel with some lead content.
For 80/87 engines with a proper STC, you can run a mixture of, say, 25% 100LL and 75% mogas. This will give you the same lead level as the original 80/87 fuel.
I know people get tired of the "follow the manufacturer’s/rebuilder’s recommendations" line. But there are enough model to model variations that it is very critical to follow whatever recommendations the people who are warranting your particular engine give.
Reprinted with permission from: General Aviation News