Lycoming cylinders generally have earned a good reputation for reliability and longevity. But in owning my Lycoming O-360-A4K-equipped Grumman Tiger for over 27 years, 14 cylinders have come off the engine—all for exhaust valve problems. The question is no longer what I am doing wrong, but instead what is happening with these Lycoming cylinders?
Over-leaning was at the forefront of the experts’ misplaced early assumptions since the problems began after Hurricane Katrina and the huge jump in 2005 avgas prices. However, for the first eight years and 1,697 hours, I never had cylinder issues. Then in 2008, I had a field overhaul done by Hagerstown Aviation and the cylinder issues began. I replaced five cylinders in the next 1,600 engine hours.
In 2017, Penn Yan Aero overhauled the engine. Having a reputation as one of the top engine overhaul shops, I assumed my cylinder problems would go away. I was wrong. With frightening regularity, eight cylinders were replaced in 1,550 hours. The first set around 700 hours SMOH and by 2023, I had replaced all four cylinders again.
Time for Savvy Aviation’s Take
My frustration grew over time and every expert I knew, including Mike Busch’s Savvy Aviation, looked into the problem. The first cylinder to go bad after the 2017 overhaul displayed a burned and cracked exhaust valve. Penn Yan inspected that cylinder and told me the cylinder was “pristine” except for the burned exhaust valve and to keep operating the engine the way I had been.
Savvy Aviation said the same thing after analyzing the trend data from the aircraft’s JPI EDM 830 engine monitor. Savvy’s response: “We could find nothing wrong in the data.” They initially suspected that I was over-leaning just as Penn Yan did, but both ultimately agreed upon inspection and data analysis that leaning was not causing the problem.
Still searching for answers, I tried the FAA approved TCP fuel treatment for three years (from 2020 to 2023) thinking maybe lead deposits on the valve, seat, or stem might be the culprit. The TCP additive did not help, suggesting that lead was not the problem.
Compression Check: More Bad News
In April of 2023, prior to my 1,300-nautical mile trek to FletchAir Fleet Support in Fredericksburg, Texas, for an annual inspection, I had an engine compression check done locally in case I had bad cylinders. It was that two Lycoming cylinders had compressions around 65 psi—both had around 650 hours total time. A 65 psi compression reading on a Lycoming cylinder is acceptable (but it’s getting low) so I left cylinder 1 and 4 on the engine. I replaced cylinder two due to zero compression at 770 hours.
During the annual, Grumman expert David Fletcher said we needed to replace both the Number 1 cylinder (it had 900 hours of time) and the Number 4 cylinder (it had 760 hours of time) because they were in the low-60-psi range. Despite having failed the compression check, David still did the valve wobble test per Lycoming SB 388C. Both cylinders failed the wobble test, meaning the valve guides were worn beyond operational specification.
Having eliminated everything else over the years, the valve wobble failure pointed to lubrication issues with the valve guides.
What About Oil Pressure?
For the first eight trouble-free years (2000 to 2008) with no exhaust valve issues, accumulating 1,697 hours on the engine, the oil pressure was always at the very bottom of the gauge’s red arc, or the high end of the allowable range. I questioned my mechanics on this as that reading seemed excessive, but they never lowered the pressure into the middle of the green arc. Beginning with the 2008 overhaul, the oil pressure was always in the middle of the green arc, which would seem to be the correct setting. However, it was the beginning of the valve issues. After the 2023 annual at Fletcher, I had the oil pressure turned up to the bottom of the red arc again.
At the May 2025 annual, I expected I may have to replace a cylinder or two based on past history and time-in-service. Much to my surprise, with three of the four cylinders at around 600 hours total time, the compressions were 75, 76, 79, 80. Then, in November 2025, Savvy Aviation expert Dave Pasquale borescoped all four cylinders. The three Lycomings that had about 700 hours since new all passed the borescope inspection with flying colors—at a time when I expected them to be failing. Pasquale commented that he expected the 700-hour Lycomings to be in worse condition than what he observed.
At this point in time, with 2,200 hours on the engine’s bottom end and 700 hours on the cylinders, only time will tell if the higher oil pressure changed the game on exhaust valve failures.
Lycoming Said: Not Much
As frustration grew over the constant valve failures, I drove to the Lycoming factory in nearby Williamsport, Pennsylvania. I sat down face-to-face with two engineers who had no answers, only blame. They said the failures were due to running with high cylinder head temps. Later that day I sent them the JPI EDM 830 engine monitor data. There were no CHTs above 420 degrees, and most running between 380 and 400 degrees. I never got a reply.
Is this a widespread issue? I certainly don’t have statistics to back that up, but AvBrief’s Smart Aviator is preparing a cylinder satisfaction survey to learn more. I do know that over time I’ve noticed more field reports of cylinder failures on Lycoming engines at much fewer hours than engine TBO. Wondering what the experts routinely observe, Doug from Columbia Aircraft Services in Bloomsburg, Pennsylvania, an overhaul shop that once did my engine overhaul, told me that most O-360 cylinder failures he sees are due to burnt exhaust valves, although Lycoming cylinders on O-320 engines may be prone to cracking.
Penn Yan Aero suggested the failures may be from metallurgy and production changes by Lycoming, as this shop has been seeing an uptick in failures. Additionally, I recently consulted with a buyer needing guidance on the purchase of a 1979 Grumman Tiger. The engine had 650 SMOH but had a top overhaul at 550 hours SMOH. Like me, that engine had low-time cylinder issues.
Final Thoughts
I’ve been chasing exhaust valve failures on my Lycoming O-360 engine for almost two decades. All the experts I enlisted could find no operational reason for the failures. Unfortunately, no other mechanic had ever done the wobble test on all the previously failed cylinders, which might have helped corroborate the lubrication issues.
This is only a single sample, but I think any O-360 owner may want to keep an eye on oil pressure—especially if there are exhaust valve issues. Borescope the cylinders every couple hundred hours (or at least at annual inspection) to check valve condition, and of course do the valve wobble test on any failed cylinder.
I’m an A&P with experience in engine overhauls. Most mechanics know that compression checks, while required, are not so reliable. You are doing borescope inspections and that is going to be a more reliable indicator of cylinder health – IF YOU KNOW WHAT TO LOOK FOR.
Mike Busch has printed a colored display of photos taken with a borescope that is very useful in understanding what you are seeing. You can find it online.
It would take a more detailed look at your exhaust valves for me to speak with high confidence, but I suspect your exhaust valves are not rotating. Each time the rocker arm pushes the valve off its seat, the entire valve should rotate such that during a minute of engine operation, an imaginary line used to index the valve bottom against the valve seat would show some rotational movement.
This movement is driven by the “Rotator” at the top of the valve stem. I suspect that your rotators either were not installed or were too loose to cause the rotation. Visit your local FSDO to obtain a list of approved cylinder repair and overhaul shops in your area. Pick one and have them look at your engine with rocker box covers removed.
Interesting you mentioned rotator caps as that is one of the things Bill Scott and I researched. Unknown to almost everyone, on Lycs the rotator cap does not physically rotate the valve but instead allows for rotation. To explain, although the cap appears to sit on the end of the valve stem, this is an illusion. The cap actually sits on the valve keepers with about a .004 inch gap between the end of the valve stem and bottom of the cap. Thus, when the rocker arm activates the valve, the first .004 inch of movement pushes down on the valve keepers, thus slightly releasing the valve. Only then does rocker arm motion start valve movement. In this released configuration, the valve is free to rotate while the mass of hot exhaust air passes around the valve head on its way to the exhaust port. It is that flow which actually rotates the valve.
BTW, we found no correlation between rotator caps and the cylinders which primarily experienced premature exhaust valve failure. The only correlation was that the odd numbered copilot side cylinders were having the most distress and their rocker boxes received considerably less oil than did the even numbered cylinders.
Bill Marvel and Bill Scott, notable among Grumman flying segments, worked this notion of oil pressure and oil flow in Lycoming engines as related to valve failures in the 1990s. Their findings, albeit airframe-specific, had some data that could not be ignored; Lycoming 360s in a Grumman airplane ate up cylinders at higher rates than the very same engine in a PA-28.
Valid arguments can be made for cooling, oil pressures, oil flow rates, valve clearances, operational characteristics, and the technical expertise of the Grumman folks and the few who specialize in maintaining those airplanes, but, clearly, something was amiss.
But something is “amiss” with Mooney airplanes, too. The Lycoming IO360 (angle valve) will run the #2 cylinder out of round during operation, to the extent that wear patterns in this cylinder are significantly different than those in the other three-and, it affects oil consumption. Not so with the same engine in the PA-28R. Go figure.
Many years ago, I had the opportunity to qualify and quantify oil flow rates in a Lycoming O-360 in a test cell configuration. The purpose was to measure how long it took for oil to reach the cylinder rocker box area and to measure the amount of oil flowing back to the sump via the drainback tubes. In no way were these tests suitable for thesis presentation, but the walk away impression was revealing: It takes longer than one might think for oil to get to the valve springs, and the amount of oil is not only small, but varies by cylinder position.
But those findings mean nothing really. Lycoming engines of this model are Type Certificated on a test stand, not in a Grumman, or a Cessna or on any other airframe engine mount.
If raising the oil pressure eliminates the insult of frequent cylinder changes, so be it. If burning shop rags in a hangar-ramp fire helps, do that too. But my personal opinion is that the Lycoming engine exhaust valve should run “oil cooled” as found in its TIO-540-AF1B installed in a Mooney TLS. Now that’s the way to fix a problem.
Thanks Paul, I had some direct discussion with Bill Marvel about this issue at the 2022 Grumman convention.
Bill Marvel here. Paul Brevard’s last two sentences are spot on. It’s been 30 years since Bill Scott (engine shop owner in KY) and I did the deep dive into this problem. It was extensively publicized in TBO Advisor, Light Plane Maintenance and a couple of other publications. We researched hydraulic valve lifters and oil flows in different aircraft and automotive engines. And we developed a simple test kit where aircraft owners could determine how much oil went to each of the four cylinders in a Lycoming O-360 engine. Way too much to unpack here from memory, but here are the high points:
BOTTOM LINE: Set the oil pressure at the max allowable below red line. Oil is needed for exhaust valve cooling and there is very little oil going to the rocker boxes in all Lycs. High oil pressure alone will increase valve longevity because it carries heat from the valve guides to the oil cooler at the same time that heat is being dissipated through the cylinder’s cooling fins. 200-degree oil on an 800-degree valve stem is about like pouring 33-degree water on your 98.6-degree body.
Hi points:
1. Due to the engine’s design, the copilot side jugs (odd numbers) get far less oil to the rocker boxes, and hence valves, than do the pilot side ones (i.e. one cubic inch in 7 minutes of in the green oil temp and pressure). Number 1, 3 and 5 jugs have more valve problems than do the others for that reason. Oil is an important coolant in addition to being a lubricant.
2. There are only two flow paths for oil to reach the rocker boxes — through the hydraulic lifters and around the tappet bodies. Almost no oil goes through the lifters by design. Oil flow around the tappet bodies is a function of oil pressure. That’s why pressure needs to be set high.
3. Mission is important. Hardest on valves are long trips at cruise power and leaned mixture because valves are marginally cooled due to low oil flow. Regardless of CHT, valve temp can become excessive and result in compression loss (50/80) at 300 hours in the worst cases we saw. Engines used for pattern work or shorter trips fare much better because they heat up and cool down. Cross country valves fry without sufficient oil to aid in their cooling.
4. Angle valve cylinders have much larger cooling fin area as well as longer valve guides and both of these factors improve their exhaust valve’s longevity.
5. Cylinder head temperature, within published limits, does not seem to affect valve longevity.
6. Our investigation revealed that in newer aircraft, red line oil pressure allowed by Lycoming was increased. I recall this was also done in Robinson helicopters. I can’t tie the two directly to our findings but it seems curious if not suspicious that the change followed what we had discovered.
6. As Paul Brevard said, this problem was fully solved with oil cooled valve guides in the Mooney TLS (AKA Bravo). It has the same cylinder heads and valves as does the Grumman Tiger, Piper Archer and many other aircraft. Yes, it is turbocharged so CHTs would be higher. But if oil cooling its guides to dissipate heat solved the problem for that engine, it stands to reason it would also for every other aircraft with the same problem.
SUMMARY: For an intermediate band aid fix, dial up your oil pressure to the maximum allowed!
The oil choice may matter also. Like other mechanics, I tend to use the commonly available Aeroshell 100. But after a series of stuck exhaust valves at the few hundred hour mark, and low compression at 90 hours total time, it was pretty clear that high temp FL operations was causing a lot of carbon buildup. A switch to Aeroshell 15W-50 with Camguard seemed to clear things up. In other words no more problems after the switch.
Since about 2011, I used Phillips X-C 20-50 with CamGuard added.
That seems to be a favorite among experts and pilots alike. I also switched to this combo in my personal plane. I’ve had no issues. But my reason for switching was partially due to oil consumption with the 15W-50 in my older engine.
This is such a common symptom with too much propeller pitch that Even GROK AI gives over a hundred references:
Just One Inch too much propeller pitch causes valve failure Lycoming and Continental have known this forever. As the old saying goes… “RPM Is Your Friend”.
If you don’t want to re-pitch your propeller then consider a controllable pitch option. MT Propeller has an STC SA01658NY. This is concerning to me because Lycoming is not in the wrong here. There’s a lot of flight schools and commercial operators running IO&O- 360s that are not burning up cylinders. The one thing they have in common is they desire take-off performance. Other words ‘Climb Pitch’.
The AA-5B Tiger has less then ideal airbox system design so, a propeller pitch that works on one AA-5B may not work on another. A faulty carb. heat valve wouldn’t be much of a surprise either.
A non-specific complaint of “exhaust valve failures” is of no use to the community. Mr. Reed replaced 14 cylinders, but according to the narrative, two failed a wobble check and one displayed an obvious seating failure. The other eleven were apparently a lost opportunity for forensic examination.
We need data, physical or statistical, not more speculation.
I agree, lost opportunities and I’m paying the bill over & over. Poor work by the maintenance community.
Quite right, Mr. Horton. There are simply too many variables in this issue of repetitive cylinder replacement to be of meaningful assistance. What we do know, applicable across all engines and installations, is that heat (general and localized) in air-cooled engines is detrimental to exhaust valves and guides. That is an indisputable fact. How that heat is managed by engineers at the engine and airframe level is as important as how its handled by the owner/operator.
Nevertheless, Mr. Reed’s unfortunate experience is difficult to digest. Imagine if he were someone new to the hobby of flying. What if this was his first airplane? What a miserable introduction to general aviation.
I would bet you that Textron Lycoming has the data you and I would want, but it’s very unlikely that file will be shared.
I agree 100% with the heat issue. What I had trouble digesting was that the cooler front cylinders (360-380 CHT) failed as regularly as the rear cylinders that ran 400-420.
For almost a decade, I thought this was an isolated issue for me and my engine. One of the main points of the article is to inform others that exhaust valve failures can be an issue – one that it is not an isolated issue. Had I known that way back when, I might have done things differently like the valve wobble test on every bad cylinder.
Cylinder issues have also emerged as a bane of the IO-390 series that have been installed in the G6/G7 Cirrus SR-20s that I have owned. It seems that the “bulletproof” reputation that is well deserved in the smaller four-cylinder Lycomings might not scale with size.
About 15 years ago I bought 8 new cylinders from Lycoming for a special performance project. I disassembled them to do my own porting work, inside and out. On the outside, between the fins is a large amount of casting flash that can be removed to improve airflow between the fins especially in the center between the valves (which is the most important area for head cooling)
When I removed the valves I noticed that the width of the valve seats was much wider on the intake valve than on the exhaust. The width on the exhaust seat was only ~0.040″. Every automotive engine nerd knows that this is a prescription for failure, especially on air-cooled engines.
Also: The oil flow to the valve area is effected by the bleed down rate of the hydraulic lifters. If the lifter does not bleed down enough, the valve is slow to close, minimizing contact pressure to the seat.
After grinding my own seats, making my own baffling and improving the oil flow (by designing and machining my own accessory case), my Lycoming seems to be quite bullet proof even at well over 230HP.
I have to overhaul my engine within the next 18 months. David Fletcher will buildup my new engine. He loves the Ly-Con port and polished cylinders. Seems like that cylinder may help improve my longevity. This time the engine will likely out live me as I’m soon 71. Hahaha.
Curious if you considered using Superior cylinders instead to see if the issue repeats itself. The wobble test is not required on them either.
I second this question. Superior claims: Millennium cylinders are known for high quality, better cooling designs, and good performance. (Note, i’m not an A&P, barely 100 hrs on a Tiger)
Yes. The last cylinder I put on is a Superior Millenium. It has 300 hours and is failing due to “washboarding” of the cylinder walls by the rings. Superior did send me a replacement cylinder under warranty.
Did anyone check the calibration of the oil pressure gauge?