Posts: 293
Joined: Sun Jun 17, 2001 1:25 am

Engine Leaning

Wed Dec 04, 2002 11:00 am

Tech Gurus,

A good friend's airplane will need to have 3 or 4 of its cylinders topped because the exhaust valves are leaking significantly. It's a Continental O-470 (six banger), mid-time (650 hrs) since overhaul to new limits and Millenium cylinders. Last year's compressions were perfect, with only 150 hours between then and now.

The aircraft (a 1964 Skylane) is actually shared between 4 folks so I'm wondering...could it be that one of them is leaning the engine too aggressively? Can overly aggressive leaning cause exhaust valve problems due to high EGT? If so, can it develop that 150 hours?

For you piston flyers, what leaning technique do you use? If you lean agressively, do you experience shorter than normal cylinder life? Anyone have any experience with Millennium cylinders?

As always, your insights are greatly appreciated.


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RE: Engine Leaning

Wed Dec 04, 2002 11:45 am

I follow the manufacturer's recommendations, which for the aircraft I fly typically means flying on rich mixture below 3000 ft pressure altitude. If I'm operating above that altitude, I gradually lean the mixutre until it starts running rough, and then I enrichen it slightly above that point. The process of determining the correct mixture is easier if you have an EGT gauge. After I start the engine and before I taxi, I lean the mixture a little so as to not foul up the spark plugs and then enrichen it before takeoff. (I fly out of sea-level based airports; I would lean the mixture for max RPM as appropriate for high-altitude airport operations.)
Last year 1,944 New Yorkers saw something and said something.
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RE: Engine Leaning

Wed Dec 04, 2002 12:06 pm

The possible answer is, yes, they may not have been leaning aggrssively enough. It's okay to run lean of peak (assuming your engine runs smoothly LOP; some flat GA motors won't because of uneven fuel distribution -- radials typically run well LOP), or significantly rich of peak, but it's bad to run AT peak.

150 hours is unusual, though. That would require some serious mismanagement.

Note here: I'm assuming you've validated that it's leaking past the valves and not past the rings. Leak down through the rings is okay as long as oil consumption isn't high. Sometimes A&P's don't check; they get repeatable low compression, and just pull the jugs.

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RE: Engine Leaning

Wed Dec 04, 2002 4:21 pm

The O-470 in 182's is notorious for this problem; our club's 182 has 1700 hours on the engine, about to be overhauled, and has gone though 5 of her 6 jugs in the past 2 years. A friend of mine who flew jumpers in a 182 babied the hell out of the engine, and still was unable to get the damned thing anywhere near TBO without replacing 2 or 3 jugs.

Also, in regards to leaning, what kind of engine monitering equipment do you have? Single or multi-port EGT? If it's a single port, you may be running fine on THAT cylinder, but right near peak on another. Ours has an OLD 6 port EGT (it looks like a regular one, but has a little dial on one side that you can use to select which cylinder to moniter) and I've seen as much as 250 degrees between the hottest and the coldest cylinders.

The science of leaning is WAY to complex to go into here in any depth, John Deakin has written a LOT about it on AvWeb, it's worth an hour or 3 to sit down and read his columns about engine operation, LOP, ROP and where it's most likely to damage the your engine.

What I usually do, BTW, in our O-470 powered 182 is after my power reduction on takeoff (to 24/24) give the mixture a few cranks back, and check all the readings, they're usually still WELL (>300) rich of peak. I continue to give it a few twists every 1000 feet, keeping an eye on the EGT and CHT, if either one gets too hot, I enrichen. When I level at cruise, I set 65% power (usually), and lean to 100 ROP on the hottest cylinder. This isn't the most economical way to run the engine, but with the fuel distribution and varience in temps (and only a single port CHT) this is the best way. If it were up to me, I'd have gone with an IO-470 with matched GAMIjectors and one of those neato digital EGT/CHT gauges, and run her like Deakin recommends.
Happy contrails - I support B747Skipper and Jetguy
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RE: Engine Leaning

Thu Dec 05, 2002 4:28 pm

Illini, if you have a 250 degree split in CHT between cylinders then your engine might have severe baffling problems. This is a system that is commonly overlooked.

Geo, I would be screaming "warranty" if I only got 150 hours out of a jug. Also, how many hours on the mags? Internally the mags could be out of adjustment or just flat wore out to the point that the points aren't spreading enough to make a good spark. Are you using "massive" type plugs that are fouled or gaped wrong? Are you using "fine wire' plugs that are eroded? How old is the wire harness? The point is that your engine might have an ignition problem.

As much as I like to blame pilots for abusing equipment, I like to look for a mechanical fault. Airplanes are pretty tough and can take some abuse without suffering too much damage.

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RE: Engine Leaning

Fri Dec 06, 2002 2:40 pm

Thanks to all of you for your very thoughtful responses. The writings of John Deakin that Mike mentioned are absolutely phenomenal. Finally, I feel like I understand a little about the subject of engine management. Interestingly, it is his opinion that the problem with Continental engines eating up their jugs is related more to manufacturing and mechanical issues than it is to pilot technique, as Chief alluded to.

Yes, it is the valves that are leaking. The 150 hours is only since the last inspection, and as I just discovered, the Millennium jugs themselves have around 900 hours on them since new, when the engine was major overhauled in 1997.

According to Deakin, the issue is most likely with poor lapping of the valves during manufacture or overhaul, or poor valve guide/valve stem alignment. This results in areas of the valve that do not properly contact the seat to enable proper cooling, and eventually the valve warps, or worse, breaks and the top gets sucked into the cylinder. We all know what comes next after that happens...the pilot gets to build some glider time. Surprisingly, CHT, EGT, and leaning really don't have all that much to do with it, unless at the extreme, of course. That's not to say that proper engine management isn't critical, just that other factors have a greater impact on the life of the engine.

Chief, I really hadn't given much thought to the mags, but the mechanic mentioned that one mag has a pretty good drop- more than it should. He also said that the plugs are at their life expectancy limit. Dunno what type they are. Have you experienced significant problems with engine durability when the plugs were worn?

What's so hard for me to fundamentally understand, is why aircraft engines are so much more sensitive than other air-cooled piston engines. How can a VW bug or a Porsche Boxster run every day, accumulating perhaps 200,000 miles, running for 6000 or 7000 hours at 3-5000 RPM, with absolutely no attention given to mixture or CHT, without ever encountering valve problems? But a $30,000 Continental can't run 650 hours? Is the quality of workmanship and materials in American made aircraft engines really that bad?

Anyway, for those who are interested, here are 3 links to Deakin's articles that are WELL worth a read, if you fly or maintain piston-driven aircraft. Great stuff, and thanks again all.

Mixture Magic:

Fried Valves:

Manifold Pressure Sucks:

Putting it all Together:

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RE: Engine Leaning

Fri Dec 06, 2002 3:11 pm

Wait a minit, here. I'm not an A&P or any kind of a mechanic but I have been a pilot for almost 43 years and what you have described sounds a lot like repeated shock cooling is eating your engine alive. Anybody in your group of 4 known for flying way up there and then dive-bombing to pattern altitude?

Illini_152: I used to fly for my college's skydiving club and I would say that it is almost universal that the pilot does a high-rate descent after dumping out the passengers. Got to get back on the ground to pick up the next load of jumpers. Just think of how that engine gets whanged by temperature extremes; a takeoff is followed by a high power climb to jump altitude which is then followed by slow flight at high power to maintain altitude and is all culminated in a closed throttle descent.

Yes, sombody may be a little chinchy on the fuel bill but the 182 usually rebels aginst that by showing you a rising CHT that prompts you to richen it up a bit, even if you have cowl flaps open. My semi-educated guess is that someone is repeatedly playing Stuka when they descend from cruising altitude.
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RE: Engine Leaning

Mon Dec 09, 2002 3:34 pm


Intersting point. The aircraft is located in Colorado, and it makes semi-frequent trips over the Rockies, at all times of the year. The other owners have oxygen and will run it up to 16,000 ft. to be able to fly direct without kissing the 14,000 ft. peaks.

However, they are also rather experienced pilots with a few thousand hours each and are adamant about making sure the other members treat the engine carefully. I don't suspect them of mistreating it in this way. Still, an interesting point to consider and I thank you for raising it. I'll pass it along to the group.

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RE: Engine Leaning

Mon Dec 09, 2002 6:24 pm


Good point about shock-cooling and skydivers, atleast in my case, however, both the operator and pilot were paranoid about shock-cooling. At least at that outfit, climbs to jump altitude were made at full power, followed, however by a highspeed powered descent.

Their primary pilot was the same guy that checked me out in our club's 182, and a friend too. He stressed gradual power reductions, and to avoid rapidly cooling the jugs, to the point of whenever possible, not fully closing the throttle until into the flare.

BTW- interesting read on shockcooling as well. Also, burnt exauhst valves isn't something I've ever heard associated with shockcooling.

- Mike
Happy contrails - I support B747Skipper and Jetguy
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RE: Engine Leaning

Wed Dec 11, 2002 3:06 pm

Shock-cooling is usually a condition resulting in a fast climb and rapid reduction in power causing a loss of cooling air over the cylinders. This mostly affects turbocharged engines that are greatly dependent on cooling from the supply fuel. On "normally aspirated" engines this is not normally a factor.


How many times have you had your car engines performance enhanced by repairing the ignition system?

Worst case scenario,,, 500 hours flight time, $1500 for mags, $500 for plugs, $500 for harness. That's $5 per hour, you spend a min of $12 per on gas!!!! How cheap can you get for some peace of mind, let alone pure fun!!!!

Don't screw around, have fun, be safe, pay your share. Air is free, gas and electricity cost $$$

Oh, your mechanic isn't free either, but neither is your Mother.
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RE: Engine Leaning

Sat Dec 14, 2002 2:33 am


Neither I nor the guy who owns a share of the plane would even think of sacrificing safety for the sake of economy. I wasn't asking about the mag/plug life in order to squeeze more life out of them. The plugs have already been replaced. Rather, I was trying to find out what sort of effect neglecting these might have on the engine, and whether or not the valve problem could be related, or the result of worn out plugs/mags.

In general, the questions I have asked are so that he and I can gain a better understanding of the factors that significantly affect cylinder and engine life. We as pilots of this aircraft do not pretend to be mechanics, but it does make sense for us to understand some of the technical details, so that we can operate and maintain it properly between annuals. The central idea being to improve safety, while at the same time not wasting money.

I also wondered about the relative durability of aircraft engines vs. horizontally opposed air-cooled automotive engines. Seems a fair question to me. Not sure which of these subjects set you off, but I didn't mean to offend.

Have a great weekend,
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RE: Engine Leaning

Sat Dec 14, 2002 12:47 pm

Addressing your rhetorical question:

"How can a VW bug or a Porsche Boxster run every day, accumulating perhaps 200,000 miles, running for 6000 or 7000 hours at 3-5000 RPM, with absolutely no attention given to mixture or CHT, without ever encountering valve problems? But a $30,000 Continental can't run 650 hours? Is the quality of workmanship and materials in American made aircraft engines really that bad?"

Think about it... a car engine, other than those brief periods when it is being accelerated at maximum power- almost never- loafs along producing afraction of its maximum power, typically 8-15 horsepower.

An aircraft engine is run at 75-100% of its rated max, ALL THE TIME.

If you look at the relative size of the cranks, bearing surfaces, etc. of aircraft engines, compared to auto engines, you'd realize how stout they are and why there aren't many car engine-based aircraft engines. They are simply not up to the task.
...three miles from BRONS, clear for the ILS one five approach...
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RE: Engine Leaning

Thu Dec 26, 2002 2:44 am

Sccutler, automotive manufacturers are up to the task in the area of innovation, increased performance and fuel economy. Lucky for them that they don't get sued for mega millions of dollars every time an engine fails!

Aviation has fuel injection but not electronic timing. Why? Because of litigation. It sucks that the technology exists to improve performance, economy, and reliability in the already robust engines that exist in GA aircraft but nobody will do it because of being afraid of getting their butts sued off!

Why do a set of mags for an O360 cost $1000 when I can get a distributor for the 350 in my Chevy for $100?

Okay, sheer volume for one instance, but legal litigation is the prime factor in the price structure of aviation related components.

Darn it, here I am ranting about stupid laws and lawyers on Christmas Day.

Happy holidays everyone!

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