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.