Mike Busch To Probe Unleaded Fuel Valve Issues


Savvy Aviation master mechanic Mike Busch is questioning the methodology of tests done by the University of North Dakota that resulted in it abandoning the use of Swift 94UL and going back to 100LL in its training fleet. The school made the switch back to leaded fuel last fall because it detected significant valve seat recession (VSR) in the Lycoming engines in its Piper Archers. VSR occurs when the valve seat erodes and the valve sinks deeper into the seat over time, possibly resulting in burned valves.

UND and Lycoming have not discussed the test results publicly but Busch, in his regular online newsletter, speculates that UND’s measurement of dry tappet clearance, the distance between the tappet and the top of the valve, may be imprecise. “A decrease in dry tappet clearance might indicate recession, but it might also be caused by other things, notably failure to ensure that the tappet body is totally devoid of liquid,” Busch wrote. “As one experienced engine builder told me, ‘There’s really no such thing as a dry tappet.’”

Busch said if UND has found that unleaded fuel causes VSR it will cause a serious wrinkle in the transition to unleaded fuel. He said the phenomenon was unheard of in engines using all types of unleaded fuel until UND detected it. Busch said that in the absence of available data from the UND Archers, he’ll do his own analysis on AOPA’s Beech C55 Baron, which is now flying on freshly overhauled engines, one burning 100LL and the other General Aviation Modifications Inc.’s G100UL, which is being touted as a drop-in replacement for avgas. He said his company will be doing the maintenance and data monitoring of both engines over the next couple of years and will be precisely measuring VSR down to the one-thousandth of an inch.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.


  1. “…UND’s measurement of dry tappet clearance, the distance between the tappet and the top of the valve, may be imprecise.”

    Please, it is the accuracy of the measurements that is in question, not the precision.

    Precision is how closely you measure something. You might measure the same dry tappet clearance with successively finer gauges as 0.3″, or 0.29″, or 0.294″ – with each measurement being successively more precise.

    But if the tappet isn’t dry, you’re simply getting an answer that is more and more precisely inaccurate.

    [Thank you for feeding my pet peeve.]

  2. Fine if they want to give it a whirl, but this is a stunt, not a research project. A single trial with a completely different engine isn’t going to produce a usable result that will move the needle. I don’t know what UND was seeing, but I ran a collegiate flight training program with 50+ airplanes for a long time and can tell you we saw a LOT of one time events that weren’t statistically significant. We also a lot of strong signal in SDRs that the manufactures said couldn’t happen. Large collegiate programs have the benefit of a large fleet of similar aircraft, and the also have researchers on campus that know what to do with that data. As far as fuel, I’ve commented before, we had valve issues with our brand new Archers, as well as with our remaned O-320 engines all on 100LL. We traced the problem to the flow setup on the new Avstar carbs the OEM was installing on all engines. They were running much leaner at the top end. Even after admitting that the flow specs were different than the older carbs (leaner) and being presented with documentation of dozens of cylinder replacements due to heat related issues (including some advanced imaging and metallurgical analyses), the manufacturer made no public changes. We swapped out the carbs with older, overhauled units and the problem vanished overnight. So I have no doubt that they were having problems, and if the problems started occurring fleet wide after switching to 94UL, that’s a signal they can’t afford to ignore. Although, since we had similar issues with the same aircraft on 100LL, their problem may continue. If it does, hopefully they’ll declare 94UL innocent of the crime. The FAA and industry have done plenty of research, but a collegiate flight school can easily do 30k-60k hours in a year so they’re likely to find a problem first, and hopefully responsibly report.

  3. Unleaded fuel has been run in Lycomings, Pratts, and Continentals for decades with no issues, but with unleaded giving longer cylinder life than leaded fuel, and no valve problems.
    80/87 where I live, did not have lead, which was allowed, but not needed to achieve the octane.
    Engines run on that unleaded 80/87 with no problems started to have sludging and valve problems when they were forced to use 100″LL”. Sludging in Continentals; valve problems in Lycomings.
    Scandinavia has been running unleaded Avgas for decades; no problems whatsoever.
    Lead is not needed for engines; lead is not good for engines; that has been long proven.
    IF UND was having a problem; the problem was not lack of lead, it was either something in the fuel; something in the test process, or something in the mind of someone.
    GAMI knows more about engines and combustion than any other group; doubtful that they put out a product with a problem.
    Good to know that Busch will be looking at the problem in an objective manner!

    • Point to note: Swift 94 is NOT a GAMI fuel, it is a different competing brand. Both are unleaded, of course, but the GAMI is 100 octane.

  4. Brian According to the ASTM D910-11 80/87 octane fuel is REQUIRED to contain lead with no execption . Level required 0.13 to 0.14 g TetraEthyl Lead per Liter. 100LL is required to have 0.43 to .045 g TEL per liter. If there is NO lead it cannot be sold as aviation fuel. if it does not meet the ASTM spec and you run this in a certificated engine you have operated outside the FAA regulations.

    Lead is Very important for Valve lubrication and cooling BUTit can be substituted and has been in auto fuels for now since 1972 when its use was mandated for compatibility for all new engine construction in the US. (and later the world.) The use of stellite hardened valve seats along with various additives has ended valve recession. If there was an issue in the study measuring valve stem clearance is an indirect and from an engineer viewpoint not a good indicator of recession .what is a good indication is to have pulled the cylinder and measure the diameter and angle of the valve seat. IF you want to measure valve eat wear and recession then that is what you measure.

  5. Mine and other glider operations have been running 91 octane ethanol free unleaded automotive fuel in our PA-25-235 towplanes for many years and thousands of hours. The O-235 engines in these aircraft typically make and exceed expected overhaul hours without issue unless someone runs a prop into something. This is tough duty – max power climes followed by quick low power descents to land then repeat over and over for the day.

    • The lead/no lead controversy is similar to the debates over dietary habits. On one hand the latest “experts” are telling you so-and-such is deadly, and if you eat even a bite of it a quick death will surely follow. On the other hand, you are reading these dire warnings while eating it with your breakfast, just as you have been doing for most of your 97 years, without detectable ill effect.
      I spent years & several thousand hours putting an 0-470 well beyond TBO on regular grade unleaded mogas without a problem.

  6. I’ll stick with my admittedly biased guess based on my non mechanical experience which may be somewhat unfair and not apply here, but it’s been my experience quite often.

    Flight schools and other fleet operators don’t like change. As soon as any new experience enters their realm, they will be compelled to explain it. They will inevitably find something, label it unknown and/or dangerous, and use it as an excuse to go back to what they have years of experience and procedures to deal with. They could have just as easily found the cotton from their own rags in the cylinders or any number of things and had a similar reaction.

    Yes, my anecdotal experience is no more scientific than their processes usually are, but it does benefit from the fact I actually consider my lack of information and it’s based on years of experience watching the same phenomena rather than too few samples.

  7. Measuring valve stem height above a surface is a good representation of valve seat wear. Valve seat face wear as well. Starting with a known height of course with new parts installed. Measured hopefully to four decimals (0.0001) and not just three decimals (0.001).

    So much easier to take the measurement and less disruptive than removing the valve and valve seat.

  8. So a man with no access to the data, testing methodology, or maintenance history comes up with a conclusion based on the fact that he doesn’t think it could happen. Cool.

    • You summed up many of Mike Busch’s theories. Where he has concrete data Mike often has good information. Where he has nothing but supposition Mike is often completely wrong.

      Years ago I attended on of Busch’s symposiums at Oshkosh. He went on about a 10 minute rant on aircraft cooling and detonation. He stated detonation absolutely cannot happen when CHTs hold below 400° over a period of time and gave information on how he came to this conclusion. At the very end he finally admitted that this might not be true of aircraft with “modern cowlings” and more efficient setups such as Cirrus, Columbias, Lancairs, RVs, etc because he didn’t know much about them. In other words his entire premise was false.

  9. If I remember correctly, the UND findings of VSR were based on measurements taken AFTER the use of UL94. But, it was unknown whether they had taken any measurements of the same engines BEFORE they started using the fuel. In other words, they did not know what the valve condition was at the beginning of the process. Garbage in, garbage out.

    Incidentally, Mr. mcapocci, I am not aware of any fuel additives in modern mogas that provide valve seat lubrication. If you have such information, I would be very interested in seeing the data. The theory of lead, or any other material, providing lubrication to exhaust valve seats is a hotly debated phenomena.

    • I mean the preceding years of running engines with no problems with valve seats would be a pretty good baseline. We’re talking about an operation that puts 1200 hrs plus yearly on the engines.

  10. It’s not just the valve seats. It’s the valves themselves. As well as the cylinder temperatures, air fuel ratio, etc.

    The older aircraft I fly might have one sensor to check cylinder head temperature. But if that cylinder is in the range, and the adjacent cylinder is running hotter, who’s to know the difference?

    • You make a very good point. Most of our aircraft came from the factory with a single CHT probe on what they felt was the hottest cylinder, and one EGT probe, often put at the junction of the exhaust system from two or three cylinders. Neither one gives us the information needed to fully know what the condition of our engines really is. In my experience, flight school aircraft have not been upgraded to modern engine monitoring systems that can record engine data so the operators can download and analyze what is happening. without good information on the operating conditions of the fleet, it is difficult to identify problems until something breaks.