In a past column I gave some background on what happens during break-in of a new or overhauled aircraft engine. Since then I have received numerous questions about why leaded fuel is needed during break-in.
Unfortunately, I do not know anyone who has a perfect answer for this. My opinion is based on lab tests and reports from field overhauls and manufacturer’s reports.
An example of a lab test that we ran back in the late 1960s was one run in a 430 CID Buick engine. The engine was installed on a test stand and run for over 20,000 simulated miles at relatively high RPM and load. We monitored the valve stem height and, at the end of the test, there was no significant exhaust valve recession.
The test was repeated with a new tank and fuel lines, plus new heads that had never been run on a leaded fuel. (We found out that the original new engine had been run at the factory on a leaded fuel.)
Now after about 15,000 simulated miles, most of the exhaust valves had receded enough to take up the lash allowed by the hydraulic valve lifters.
As far as field data, about 20 years ago, a west coast oil company started to market an 80/87 unleaded avgas. This was perfectly legal since the ASTM D-910 spec for avgas specifies that the fuel only meet a maximum of 0.5 grams per gallon lead level. There is no minimum level.
A few months after the introduction of this fuel, I started to receive numerous valve recession complaints, but only from the west coast. Further investigation revealed that every failure occurred on aircraft that had operated on the unleaded fuel.
Subsequent to this I have heard numerous reports of valve recession on aircraft that have operated exclusively on unleaded mogas. Now it does not happen to every aircraft every time, but it does happen a significant part of the time.
So what is the mechanism here and what does the lead do to prevent exhaust valve recession?
This is one of those things that if you talk to 10 experts, you will probably get 20 to 25 different answers.
The most common ones are: Lead acts as a solid type lubricant to protect and cushion the valve and seat; another is that the lead alloys with the seat material; and still another is that it improves the heat transfer from the valve and heat tempers the seat.
In the Buick engine test, the low level of lead that was needed to protect the valves was due to the liquid cooling of these engines and the subsequent lower seat temperature compared to an air cooled aircraft engine.
When an engine is running, especially under high load, the exhaust valve is exposed to direct flame temperatures when it first opens. To keep the exhaust valve from getting too hot, engine manufacturers design a direct heat path up the valve stem to the guide, valve tip, and the oil flow.
But the most critical point is the valve tulip edge, and that is designed so that most of the heat is transferred to the valve seat during the time the valve is closed. There is not much time for the heat to be transferred, and the transfer depends on the difference in temperatures between the valve edge and the seat.
Since a liquid-cooled engine exhaust valve seat runs several hundred degrees cooler than that seen in an air-cooled engine, it is easy to see why aircraft engines are much more critical.
So what is the answer? I feel it is a combination of things.
First, in a new engine, the valves and seats are both ground. The surface of the valve and seat are not perfectly smooth. The micro ridges and valleys from grinding allow some leakage after the first startup, and provide a poor heat transfer path from the valve to the seat.
With a leaded fuel, the byproducts of combustion tend to coat the valve and seat, which yields improved sealing and a better heat transfer path. Over time, the seat does harden somewhat and the engine goes on to live a long productive life.
With an aircraft engine, it becomes much more critical, especially during break-in. After that a small amount of lead is required during the life of the engine to protect the valve and seat.
It is not a precise amount, but during break-in I would recommend running at least 25% 100LL for the first 50 hours. After that, a small amount every so often should keep all running well.