The Valve Seat Recession Conundrum

In a recent blog posted on General Aviation News by Ben Visser, he made a point that poses a serious conundrum for the conversion to unleaded avgas: “The big problem here is that almost all of the aircraft piston engines out there need to be broken-in on 100LL, then they can be switched over to auto gas. But if they are started on auto gas when new, they will probably have exhaust valve problems.” The exhaust valve problems that Ben is alluding to is commonly known as “valve seat recession.”

The authors of this blog want to make it clear that we are not experts on valve seat recession, and that we are only passing along information written by others that you can find on the Internet; for instance in this report on Valve Seat Recession, what appears to be an authoritative dissertation on the subject from the Partnership for Clean Fuels and Vehicles. There is much more information on the subject if you Google “valve seat recession.”

The point we would would like to make is that this appears to be a very important subject in the conversion from leaded avgas to unleaded avgas, yet we can find no record of its discussion by the Unleaded Avgas Transition Aviation Rulemaking Committee, usually referred to as the ARC committee. In fact what we do find is a conflicting stance by engine manufacturers.

Take, for instance, Continental Motor’s published stance on unleaded gasoline use in its engines as found here on its website. It comes to the startling conclusion that running a Continental aircraft engine on unleaded auto fuel can cause serious damage in as little as 10 hours: “The use of unleaded auto fuels with engines designed for leaded fuels can result in excessive exhaust valve seat wear due to the lack of lead. The result can be remarkable, with cylinder performance deteriorating to unacceptable levels in under 10 hours.”

What is odd is that experience in the field, using unleaded auto gasoline in light aircraft engines, including Continental engines, for the last three decades, clearly refutes that sweeping claim.

According to Todd Petersen of Petersen Aviation, “When Continental wants to attack autogas they make valve seat recession out to be a common problem, but then last May, the company publicly claimed that nearly all of its engines are compatible with the new ASTM 94 UL specification, which is essentially 100LL with no TEL [Tetra Ethyl Lead, the lead additive that is the LL in 100LL].  I still maintain that if an aircraft owner obtains parts directly from Lycoming or Continental, that they’d be getting new spec parts that require no lead. I’ve spoken to people who overhauled engines and had them experience a loss of compression within 30 hours, but after putting in new Lycoming cylinder assemblies, the engines were lasting just fine. So given the feedback I’ve received over the years, I still say buy parts directly from Lycoming or Continental, and it’s not a problem. Radials and Franklins though, and maybe some of the small older Continentals, will be damaged. Until I hear something persuasive saying otherwise, I have to stay with that.”  Petersen provides additional information on this issue on his website.

So which is it? Converting to unleaded avgas is a danger and is going to cause valve seat recession and engine damage, or using unleaded gasoline in aviation engines is not a problem? If you read the PCFV report, noted above, it appears that valve seat recession really depends on the head, valve and valve seat design and how the engine is operated.

Unfortunately the ARC committee’s work is done in secret, so we have no idea how the ARC is addressing this conundrum.

(Submitted by Dean Billing with assistance from Todd Petersen of Petersen Aviation)

The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.


  1. Stu Brown says

    I believe the specification for 80 octane aviation fuel was for a maximum of .5 grams of TEL per gallon. The lead was added to increase the octane level to 80. I have to assume on some batches of gas there was no lead added. I wonder how Continental rationalizes their position on unleaded fuels v.s. The specification for 80 octane avGas?

  2. Pgchelew says

    Valve seat recession was a problem experienced by other than aero engines.  Specifically, older BMW motorcycle boxer engines, air cooled horizontally opposed twins, incorporated a softer steel alloy meant for leaded fuels.  When unleaded fuel began to predominate, these engines experienced valve seat recession, with very noticeable power loss.  The solution that was developed incorporated a change of valve seats to a hardened material, both on OEM and existing installations.  There could be a parallel problem in some aero engine designs.

  3. F1boss says

    I remember way back when we all used leaded auto fuel, and the government was gonna make us switch to unleaded; geez – I must be old! Much wailing and gnashing of teeth ensued. I do not recall any specific problems associated with that change, and auto makers changed to harder valve seats and sodium cooled exh valves to insure no future problems. Since our aviation engines are already equipped with these better parts, and have been since the 30s, my expectation is we will also be OK with UL fuel in many of our aircraft applications, if we can only get rid of the alcohol. The turbo’d engines are gonna have to make some other changes, which might be handled by water injection systems, kinda like the Germans used in WW2.
    Keep in mind, we are not inventing something new!
    Sounds like we might be running into ‘marketing pushback’?
    Carry on!

  4. Kent Misegades says

    Dennis, our understanding is that new engines from Lycoming and Continental no longer have this problem.  The focus in this article is primarily on older engines.  Note also that the entire new generation of engines from Lycoming, Continental, Rotax, ULPower, Jabiru, and others are designed to operate on 91-93 AKI ethanol-free, lead-free autogas.  In at least one instance, the very successful Rotax 9XX series, autogas is the recommended fuel – 100LL causes a number of problems and should be avoided.

  5. Dennis Reiley says

    No, the problem is the engines are designed for 100LL. And the industry refuses to design them for autogas. You have to eliminate the 100LL mentality. An engine designed to run on autogas will run on 100LL. Doing the opposite is the problem and that problem is centered directly on manufacturers who refuse to redesign their engines.

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