MythBusters: Does lead have any effect on valve life?

I received a note, recently, from a gentleman who is completing an experimental aircraft with a Lycoming engine. He’s planning to use auto gas and wonders what precautions he should take.


I received a note, recently, from a gentleman who is completing an experimental aircraft with a Lycoming engine. He’s planning to use auto gas and wonders what precautions he should take.

I went through all of the common steps, including the need to break in the engine on leaded fuel and to add a small amount of leaded avgas during the life of the engine to protect the exhaust valve seats.

His reply was most interesting. He thanked me for the information, but then added that he could not believe I continue to believe the myth that lead has any effect on valve life. He claimed that lead is only added to fuel to increase the octane ? period. No other benefits.

There are two things that bother me about his reply. First and foremost is the absolute certainty of his statements. Second, in my opinion and that of every industry expert I have talked with, he is wrong.

Lead byproducts of combustion coat the exhaust valve and seat interface. This prevents the valve from grinding into the valve seat and recessing into the head.

Exhaust valve recession is very real and very easily demonstrated. I observed many lab tests on automotive engines in the late 1960s. Exhaust valve recession would occur any time an engine designed to run on leaded fuels was run at high RPM and high load with unleaded fuel. The automotive industry has eliminated this problem in today’s engines by installing hardened valve seats. In addition, auto engines are liquid cooled.

About 15 years ago, an aviation fuel supplier started selling 80/87 avgas with no lead. This is perfectly legal because the spec for 80/87 only calls for a maximum of 0.5 grams of lead per gallon and no minimum.

The fuel worked well for awhile. Then people started to overhaul their engines, and found that when they used unleaded fuel from the start, the exhaust valves recessed into the head. Usually the valves would recess enough to use up the clearance and then the valve would be held off the seat until valve burning occurred.

Here we have a problem that has been observed and documented by many knowledgeable people, yet there are those out there who call it a myth and a complete fabrication. I believe that this is one of the biggest problems ? if not the biggest ? in aviation.

I may not always be right?? just ask my wife ? but when I hear people make statements that I know are incorrect, I get concerned.

If this gentleman goes around and convinces a lot of other pilots that lead has no effect on valve life, many of them may run their engines only on unleaded auto gas. Then, when their valves burn, they will probably hire this gentleman as an expert witness, then sue Lycoming or Teledyne Continental for producing a defective engine.

The engine manufacturer may win the lawsuit, but the legal fees will be enough to drive up new engine prices even more. This will result in fewer and fewer pilots entering the general aviation community because of high costs. Then we will all sit around and discuss how general aviation is dying and wonder what we can do to keep it going.

So remember, it takes only two things to be an expert: gray hair and hemorrhoids. The gray hair makes you look distinguished. The hemorrhoids make you look concerned.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

Speak Your Mind

*