question about low compression and overhauls.

[midcap]

So, I have been reading forums and listening to podcasts and things like that on people's stories about engine overhauls. I know a common TBO is 2000 hours. but what I have read and heard is that a lot of people will have low compression at 1200 hours and then do a complete overhaul.

Do, the cranks and cams go out that often that you need to replace them or the bearings that often?

It seems like if you are running oil analysis and nothing out of the ordinary is showing up, you would just need to do a top end if compression got low.

I have heard people say, well if the top is going out then the bottom is too, that really doesn't make much sense to me.

My back ground is in high performance marine engines, 2 strokes, s/c 4 strokes etc and I have seen engines that burn up valves, cracked heads, bent valves etc, for me only to put some fresh heads on the engines and then have the engines run another 100 hours at 100% load making 120hp per liter at 5000 rpms all day long. That is really what piqued my question.

 

[tyndall]

Low compression can be caused by corrosion, which is usually caused from lack of use. If the cylinders are rusting, there's a good chance the cam is too. Replacing the cam means splitting the case, which means replacing the bearings and at that point, you are within spitting distance of calling it an overhaul.

Cams can go bad in a year, depending on where the aircraft was sitting. Cranks are usually ok or can be saved with a grind or polish. The trick is to keep it flying. Flying once a month will save you money. As long as the bottom end is kept oiled, they can usually go well over tbo.

 

[midcap]

Low compression can be caused by corrosion, which is usually caused from lack of use. If the cylinders are rusting, there's a good chance the cam is too. Replacing the cam means splitting the case, which means replacing the bearings and at that point, you are within spitting distance of calling it an overhaul.

Cams can go bad in a year, depending on where the aircraft was sitting. Cranks are usually ok or can be saved with a grind or polish. The trick is to keep it flying. Flying once a month will save you money. As long as the bottom end is kept oiled, they can usually go well over tbo.

that's what I thought. So literally if you just spin the engine over enough to build oil pressure you will get the cam and crank lubed and the cylinder walls lubed, better yet, pull the plane out and let the engine come up to normal op temp then push it back in.

I am sure your battery is going to hate you.

 

[Mike Smith]

that's what I thought. So literally if you just spin the engine over enough to build oil pressure you will get the cam and crank lubed and the cylinder walls lubed, better yet, pull the plane out and let the engine come up to normal op temp then push it back in.

I am sure your battery is going to hate you.

Not exactly, you need to run the engine at operating temperature for a period of time long enough to burn off moisture in the oil. Cant really do that on the ground. Gotta fly.

 

[midcap]

Not exactly, you need to run the engine at operating temperature for a period of time long enough to burn off moisture in the oil. Cant really do that on the ground. Gotta fly.

that makes sense also.

 

[Ted]

TBOs vary wildly. There are engines with 1200 hour TBOs. Many Continentals have 1400 or 1500 hour TBOs, and then you have Lycomings for which 2000 is a standard (but not gospel). So that can influence the decision.

For the most part, though, simply doing a full overhaul because you're doing a top overhaul is silly. I see many people spend much more on their engines than they need to.

 

[midcap]

TBOs vary wildly. There are engines with 1200 hour TBOs. Many Continentals have 1400 or 1500 hour TBOs, and then you have Lycomings for which 2000 is a standard (but not gospel). So that can influence the decision.

For the most part, though, simply doing a full overhaul because you're doing a top overhaul is silly. I see many people spend much more on their engines than they need to.

yeah I'd imagine at some point, you are spending money just to spend it.

 

[bradg33]

yeah I'd imagine at some point, you are spending money just to spend it.

That or you just don't understand what you're doing. There are a lot of aircraft owners who simply don't understand how their engine (or the rest of their airplane, for that matter) works. When an A&P suggests they repair/replace/overhaul something, they just say "OK," without doing any research of their own or asking questions, etc., and hand over a stack of cash. Same goes for reaching "TBO." Some owners don't understand, or don't care, that TBO is nothing more than a guideline. I don't feel bad for those people.

 

[midcap]

That or you just don't understand what you're doing. There are a lot of aircraft owners who simply don't understand how their engine (or the rest of their airplane, for that matter) works. When an A&P suggests they repair/replace/overhaul something, they just say "OK," without doing any research of their own or asking questions, etc., and hand over a stack of cash. Same goes for reaching "TBO." Some owners don't understand, or don't care, that TBO is nothing more than a guideline. I don't feel bad for those people.

I can definitely see that happening with airplanes.

 

[Dan Thomas]

Not exactly, you need to run the engine at operating temperature for a period of time long enough to burn off moisture in the oil. Cant really do that on the ground. Gotta fly.

Yup. An awful lot of engines are ruined by guys running them just "to circulate the oil." One of the byproducts of combustion is water vapor, and these engines, when they're not at operating temps, leak combustion gases past the rings and into the case, where that water vapor condenses, mixes with the oil, and reacts to form acids that eat the engine from the inside. The engine and oil has to get hot enough long enough to drive off the moisture.

I've taken rocker covers off after a brief run and found water in them.

 

[sourdough44]

I have a question along similar lines. Say one has a 40 y/o engine(0-360) with compression checks averaging the low 70's. This engine is also mostly stock since factory, 1800 hours time. Has anyone seen much difference, or any, after a complete overhaul or factory new engine was installed?

I'm just wondering if a 'fresh' engine has any more performance/power compared to a 40+ y/o 'tired' engine?

 

[Tom-D]

Yup. An awful lot of engines are ruined by guys running them just "to circulate the oil." One of the byproducts of combustion is water vapor, and these engines, when they're not at operating temps, leak combustion gases past the rings and into the case, where that water vapor condenses, mixes with the oil, and reacts to form acids that eat the engine from the inside. The engine and oil has to get hot enough long enough to drive off the moisture.

I've taken rocker covers off after a brief run and found water in them.

You must realize, while you are heating the engine to boil out the water, you are making water, so you will shut down with the exact same amount of water in the sump as you started with.

Rocker box covers are mostly tin covers, they cool fast, so they become a cold place for the warm air/water to condense on.

 

[tinerj]

You know, running the engine periodically to accomplish something beneficial is like flossing, where is the scientific evidence?

Despite statements to the contrary, I've seen C-150s that have set for four years whose engines checked out ok. And I've seen rust form on unpanted steel parts in three days.

I've seen guys land and take off the oil cap until the engine cools to let the moisture escape. Or, it lets moisture in. Others pull the prop through to keep everyone oiled when the engine isn't run. Others run it for five minutes. Others run it until the oil temp moves. Others say you've gotta fly the plane.

Here's what I do: Fly the plane when I want to and park it in the hangar and walk away and leave it untouched until I'm ready to fly again.

 

[Mike Smith]

You must realize, while you are heating the engine to boil out the water, you are making water, so you will shut down with the exact same amount of water in the sump as you started with.

Rocker box covers are mostly tin covers, they cool fast, so they become a cold place for the warm air/water to condense on.

So, does that mean that we are not getting rid of any moisture in the engine? If that is true, why is regular flying better for the engine? I truly don't know.

 

[Tom-D]

You know, running the engine periodically to accomplish something beneficial is like flossing, where is the scientific evidence?

Despite statements to the contrary, I've seen C-150s that have set for four years whose engines checked out ok. And I've seen rust form on unpanted steel parts in three days.

I've seen guys land and take off the oil cap until the engine cools to let the moisture escape. Or, it lets moisture in. Others pull the prop through to keep everyone oiled when the engine isn't run. Others run it for five minutes. Others run it until the oil temp moves. Others say you've gotta fly the plane.

Here's what I do: Fly the plane when I want to and park it in the hangar and walk away and leave it untouched until I'm ready to fly again.

You are nearly 100% of the pilot /owners in GA.
yet we have this bug-a-boo about flying the aircraft is the only way to prevent corrosion in an engine.

 

[Dan Thomas]

You must realize, while you are heating the engine to boil out the water, you are making water, so you will shut down with the exact same amount of water in the sump as you started with.

Rocker box covers are mostly tin covers, they cool fast, so they become a cold place for the warm air/water to condense on.

The engine gets way more water past the rings when it's cold than when it's hot. That's why ground running is bad. In flight, everything tightens up and the moisture gets boiled out much faster than it's getting in, leaving the case relatively free of water.

I see this problem regularly. Regularly ground-run engines have corrosion in them. Regularly flown engines don't.

http://www.aviationpros.com/article/10387461/corrosion-how-does-it-affect-the-internal-engine

 

[Tom-D]

So, does that mean that we are not getting rid of any moisture in the engine? If that is true, why is regular flying better for the engine? I truly don't know.

creating water in any engine is a cyclic thing, you make it as to get rid of it.
Regular flying... trying to prove that it helps is like trying to prove why it didn't fail.
the picture is of a 0-300 cylinder that sat in a unheated hangar in Arlington Wa. unpreserved for 25 years, the corrosion was so minor we could wipe it out with a oily rag. there was one lifter that had a minor pit. the engine was mid life when it wa

The engine gets way more water past the rings when it's cold than when it's hot. That's why ground running is bad. In flight, everything tightens up and the moisture gets boiled out much faster than it's getting in, leaving the case relatively free of water.

I see this problem regularly. Regularly ground-run engines have corrosion in them. Regularly flown engines don't.

http://www.aviationpros.com/article/10387461/corrosion-how-does-it-affect-the-internal-engine

The engine gets way more water past the rings when it's cold than when it's hot. That's why ground running is bad. In flight, everything tightens up and the moisture gets boiled out much faster than it's getting in, leaving the case relatively free of water.

I see this problem regularly. Regularly ground-run engines have corrosion in them. Regularly flown engines don't.

http://www.aviationpros.com/article/10387461/corrosion-how-does-it-affect-the-internal-engine

Got a reference for that? I ask because a cold cylinder has a tighter fit to the piston than a hot one, pistons heat up quicker than a cylinder that is always trying to stay cool.

 

[Tom-D]

your article says.
The purpose of this article is to help raise awareness of this problem by presenting some of the current information available including various opinions of industy experts. What is offered here is primarily based on reports already published in the field. Through this discussion, we hope to stimulate more study in the area.

it is not a definitive end all on engine corrosion.

Here is the picture I was trying to up load earlier, this cylinder sat unpreserved for 25 years here in humid salty Puget Sound. you could wipe out the corrosion with an oily rag.

 

[Tom-D]

Just saying,,corrosion in any engine has been a hot topic since Wilber asked the question. every one has an opinion as to how much damage it causes.

But like I've always said, every engine is a Pandora's box, you don't know what you have until you open it.

 

[Tom-D]

Not exactly, you need to run the engine at operating temperature for a period of time long enough to burn off moisture in the oil. Cant really do that on the ground. Gotta fly.

Do you realize that engines get hotter on the ground with out the cooling air they get flying?

 

[Tom-D]

So, does that mean that we are not getting rid of any moisture in the engine? If that is true, why is regular flying better for the engine? I truly don't know.

That is a quandary that the jury is still out. I believe corrosion in the engine is typical of the region you operate in. those in Tucson don't worry about this stuff.

 

[Tom-D]

Question for the room.

why doesn't short hop seaplane operators see more engine corrosion than any other operators?

 

[Fearless Tower]

Do you realize that engines get hotter on the ground with out the cooling air they get flying?

Are we talking about CHTs or Oil Temp though?

Yes, you can get CHTs up pretty quick by doing a runup on a hot day.

But I've never seen oil temp on any engine get anywhere close to what I seen in flight by running on the ground (well, unless I completely bypass the oil coolers on the Beech 18).

 

[Tom-D]

Are we talking about CHTs or Oil Temp though?

Yes, you can get CHTs up pretty quick by doing a runup on a hot day.

But I've never seen oil temp on any engine get anywhere close to what I seen in flight by running on the ground (well, unless I completely bypass the oil coolers on the Beech 18).

You'd not run your 985s long enough for the oil temps to catch up. hopefully.

and yes we normally don't run on the ground to do any good because the CHTs will go off the scale.

But,,,, the flat engines with the integral sumps can get the oil temps high enough to get the water out. But as said, you are making water at the same time so what good does it do?

 

[Dan Thomas]

creating water in any engine is a cyclic thing, you make it as to get rid of it.
Regular flying... trying to prove that it helps is like trying to prove why it didn't fail.
the picture is of a 0-300 cylinder that sat in a unheated hangar in Arlington Wa. unpreserved for 25 years, the corrosion was so minor we could wipe it out with a oily rag. there was one lifter that had a minor pit. the engine was mid life when it wa



Got a reference for that? I ask because a cold cylinder has a tighter fit to the piston than a hot one, pistons heat up quicker than a cylinder that is always trying to stay cool.

The piston is aluminum. The cylinder barrel is steel. Aluminum has almost exactly twice the coefficient of linear thermal expansion as steel, so the piston expands way more than the barrel. Therefore, the piston-to-cylinder clearance is larger when the engine is cold. Any engine guy knows that. With larger clearances, there is more blowby since more ring gap is exposed.

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

I too have seen engines apart after having sat for many years. They were last run in flight, not ground-run. It's not so much environmental moisture that does the damage; it's combustion byproducts, including water.

 

[Tom-D]

The piston is aluminum. The cylinder barrel is steel. Aluminum has almost exactly twice the coefficient of linear thermal expansion as steel, so the piston expands way more than the barrel. Therefore, the piston-to-cylinder clearance is larger when the engine is cold. Any engine guy knows that. With larger clearances, there is more blowby since more ring gap is exposed.

the piston will heat up faster than the cylinder and close that gap quickly. So the period you speak is a very short one

 

[Chow_Chow]

I agree with Dan, and that is the broadly accepted view. It takes time to boil off all the moisture in the oil, so it is recommended to fly at a higher power setting for at least an hour session at least once a month. All the moisture that has accumulated in the sump gets boiled off and vented out. It’s still hot as you shut down so not much new accumulates, but it’s less than what had accumulated in there over all the other short flights.

 

[Stewartb]

I had a 1200 hour Continental o-470 that used oil. Compressions were satisfactory. That engine is famous for worn valve guides, and that was my problem. With a 1500 hour TBO doing a top didn’t make sense to me so I replaced it with a reman. Everyone has their own threshold of when a top might make sense and when just doing a major makes sense. In general, I’m not a fan of a top overhaul unless the cylinders have an AD that requires replacement.

 

[Jeff Oslick]

A top overhaul at 1200 hours is not unusual for the bigger 6-cyl engines.

What is important to remember is that each of the cylinders is essentially a separate engine. Just because one has a problem doesn't mean all 6 have a problem. Or maybe they do have a problem but it could be different issues. You use a combination of oil consumption, oil analysis, and more importantly compression checks and borescope inspections to see what the problem is.

From a practical perspective, if you have the engine apart enough to remove a cylinder, it isn't much more work (maybe 30 minutes each) to remove the other cylinders on that side. The bulk of the labor is R&R of the exhaust, intake and baffle components. In the middle of that fun right now for two cyls that need ring work.

 

[Dana]

It depends on why the compression is low. Last year I had 3 cylinders with declining compressions (as low as 30/80). Around 6-700 hours inn the engine Clearly leaky valves from the hiss, borescope showed nothing amiss. Ground the valves in place, compressions back to 70s, still good a year later. Cost, a couple hundred bucks.

 

[Pinecone]

that's what I thought. So literally if you just spin the engine over enough to build oil pressure you will get the cam and crank lubed and the cylinder walls lubed, better yet, pull the plane out and let the engine come up to normal op temp then push it back in.

No and NO.

The cam in an aircraft engine only gets lube for the lobes from splash. Spinning the engine to get oil pressure will not lube them, so you are running them dry.

You need to get the engine up to full operating temperature for about an hour to drive out all the moisture. You really need to fly it.

 

[Pinecone]

Question for the room.

why doesn't short hop seaplane operators see more engine corrosion than any other operators?

Because they are flying a lot. Same with flight schools.

As has been pointed out, if shock cooling was such a big deal, wouldn't flight schools be hit hard????

 

[chemgeek]

So, I have been reading forums and listening to podcasts and things like that on people's stories about engine overhauls. I know a common TBO is 2000 hours. but what I have read and heard is that a lot of people will have low compression at 1200 hours and then do a complete overhaul.

Do, the cranks and cams go out that often that you need to replace them or the bearings that often?

There is nothing wrong as an owner in just fixing what's wrong. Low cylinder compression is a symptom of cylinder issues, and does not imply anything else is wrong. There are lots of reasons for low compression, and those can usually be identified and rectified. There is no sense in replacing the bottom end unless you know something about its history, or engine performance/visual inspection suggests a problem. You might be surprised at how long an engine will perform well with out-of-spec cams and cranks.

When my engine came to the end of its last run, the lifters were starting to spall and the cam was starting to wear significantly due to advanced corrosion. After evaluation, both the cam and crank were well beyond specs and basically junk for re-use. But the engine ran at book, if on borrowed time. Before the overhaul, I had been through several cylinder replacements. Those cylinders has a wide range of time on them at replacement, between 900 and over 1900 hours. My decision to overhaul my engine at about 1800 hours was based on history (a previous field overhaul which reused the crank and cam over 25 years prior), lifter spalling and a visibly worn cam. Since the case had to be split anyway to fix the cam and lifter issues, it was a good time to evaluate the crank, which turned out to be out of specs, so the call to do a full OH was reasonable, and gave me a chance to get all the cylinders back on the same clock, as well as install a high compression STC at no extra cost. Every situation will be different.

 

[pfarber]

Not exactly, you need to run the engine at operating temperature for a period of time long enough to burn off moisture in the oil. Cant really do that on the ground. Gotta fly.

Anytime you get the engine oil above 180f you are boiling off water. The issue is that unless you sping the motor fast enout the normal splash oiling is not reaching all the right spots.

There is no difference between 2400rpm on thr ground and 2400 rpm im the air, or 180f oil temp.

 

[Mike Smith]

I don’t think I would ground run my engine at 2400 for an hour on the ground. YMMV

 

[Dan Thomas]

I don’t think I would ground run my engine at 2400 for an hour on the ground. YMMV

Yup. CHTs will easily redline long before the oil warms up.

 

[Fracpilot]

I have an IO-470. CHTs and EGTs are great. Oil consumption is hardly any at 1 quart per 12 hours. 30 years since TBO, continued oil analysis for the last 12 oil changes. Engine time at 1700 hours. The #1 cylinder had low pressure and it was steadily declining over the past 3 annuals. I chose to just OH the #1 cylinder instead of the engine. All other pressures are fine and nothing is telling me to OH the engine now.

Mike Bush has a great write up this on when to OH an engine. Just because you’re at TBO doesn’t mean you have to OH. For me, if a part needs changing, I fix it. It doesn’t mean to spend almost 50K for an OH. I’m putting on new engine mounts, exhaust and overhauling a cylinder now. My goal is a get 2500 hours on this engine.

When engines need OH, they will talk to you. The oil analysis should pick up corrosion in higher than normal nickel and/or iron numbers. You should start to see the metal in the oil changes using 100 micron screens and cutting filters along with the oil analysis numbers changing.


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