I wish this was a comprehensive KB article I was presenting, but alas, I thought it just a good idea to start a dedicated thread on the topic of preventing engine corrosion among the various situations, as spurred by Whee's pre-heater topic. Feel free to add questions or answers or additional info as you please, maybe we will poach some info from this for an actual KB article on the topic.

For now, time to spew:

- best practices for preventing corrosion in a flying airplane with a running engine
- best practices for long term periods of inactivity
- pre-heat vs constant heat
- dynamics of rust formation
- why pre-heat? Is a really cold start (if it will fire) actually damaging to an engine?

Rust formation

There seems to be a lot of anecdotal opinion on the formation of rust on engine internals. Here's an anecdote on rust formation in 2 different environments:

1. Took my BH fuselage over to a buddy's shop to do some modification fab. Welded in some bare 4130, left it in his shop which was open air during the summer, with the standard temp swing and humidity of NW Oregon. It developed surface rust all over every bare steel surface after a few weeks. Nothing that can't easily be blasted clean before coating, but it's still rust.

2. Said BH came home to my insulated and heated garage/shop. Chemically stripped a lower cluster with methylene chloride and a stainless wire brush to add float fittings and never completed the project. It's been bare steel for almost 2 years. It's completely rust-free in an environment above 60°F.

There have been claims that constant heat on an engine actually contributes to corrosion, but that conversation opens up a can of worms with the actual dynamics in play. Combustion produces moisture in the case. Going from a hot engine to cold as it naturally cools down is going to condense some water out of the internal air, depending on the ambient temp, and with it likely some nasty byproducts of gasoline combustion.

Normally, water will evaporate and go bye bye if exposed to air, unless it's in the oil. Perhaps using one of those air circulators with a desiccant chamber during cool-down after flying would be effective? Maybe it's just overkill and more flying is in order, but that's not possible for some of us.

Some scenarios

For myself, this discussion is relevant as I have 2 engines sitting:

- A brand new 540 that I field overhauled myself, replacing nearly everything to the tune of about $30k.
- A mid-time 320 from my Pacer that has a young-ish bottom end (350 hrs) and an older top end (750 hrs.) Recently spent $1500 to rehab the top.

The 540 is stored in a warm hangar at my friend's maintenance shop. I "fogged" the internals after assembly with a high pressure misting nozzle through the holes in the case with the accessory case cover off, before bolting it on, blowing a slurry of Federal Mogul Sealed Power and Phillips XC20-50 over every interior surface. Cam lobes and lifter faces are coated in manganese phosphate. Cylinder walls are coated in black nitride. It's never been run, no acidic byproducts of combustion to contribute to corrosion. Seems like an ok scenario?

The 320 is stored in my cold hangar. Had all the jugs off for a little rehab to get new exhaust guides, valve grinds, new ring kits and a hone for crosshatching. While they were off we slathered cam lube over the lobes and lifter faces. Also been running W-100 Plus for the short life of the bottom end. To answer your question... handprop accident by previous owner took out the crankshaft, so it got an IRAN.

Trying to decide whether to pickle one, both, or none. Pickling is easy but can be messy.

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Like Z said at the beginning, most corrosion "evidence" is anecdotal...not that the process of corrosion is in dispute, but rather what reduces or exacerbates it. One guy has an engine turn to crumbs a year after installing a pre-heater, and another guy leaves his pre-heater on 24/7 for a dozen years without any corrosion. Probably more to the story than just the pre-heater?

One thing that I think confuses the issue in peoples minds is the difference between what's happening outside the engine vs inside the engine. Everyone knows that temperature affects the moisture holding capacity of air, and it determines whether moisture will condense out of the air, but anyone who's ever dried out a pair of boots also knows that airflow is infinitely more important than temperature in removing excess moisture. Without air movement you can change the temperature of the moisture, but you can't change the level of moisture in an enclosed space.

Without the ability to move air through the engine we're stuck with the (high) internal humidity level at shutdown, which is nothing like the humidity level in the rest of the hangar. Heating the engine probably evaporates some of the moisture out of the engine, but the parts of the engine not subjected to airflow from the breather tube or removed oil filler cap cannot dry out regardless of temperature, because there is no airflow to move the moisture.

So is it better to keep that moist air trapped in the engine warm, cool, or frozen?

I think how the airplane is flown also matters. The gospel is to get the engine up to temp to boil moisture out of the oil...fine. But if the pilot then shuts down after a fifteen-minute low-power descent and taxi, they've reloaded the oil with moisture. For folks in controlled airspace there might not be any choice, but given the choice I like to keep my power and oil temperature nice and high until landing to reduce the amount of moisture redeposited into the oil.

I don't have a clue whether keeping the engine warm all winter is better or worse than just heating it before flying, and for some people that question is less important than being able to fly when they want. Other than somewhat universal agreement that continually heating just the oil will often evaporate water out of the oil that then condenses on all the internal engine parts above it, causing a lot more corrosion than no heat at all, I've yet to see a consensus.

So here are the totally non-proven anti-corrosion techniques I've adopted:

•Hangar.
•25 hour oil changes.
•CamGuard and MMO...not necessarily at the same time.
•Flying in such a manner as to keep my oil temperature at 180 degrees until shut-down if possible.
•Immediately removing the oil filler cap at shut-down and leaving it off with a cloth cover so nothing falls in.
•Keeping the cowling and propeller covered in thick quilts to reduce the thermal cycling of the engine throughout the day.
•Pre-heating prior to flying, and then again after shut-down to encourage as much evaporation as possible.
•Flying as often as practical, and every couple weeks regardless of whether I even want to.

I'm going to investigate a forced-air engine breather option similar to what is commercially available, as it seems to me to be the single most effective way to get moisture out of the engine.

Years ago I had a debate here about the need to reach a minimum oil temp to vaporize moisture. Some erroneously believed it had to be the boiling point of water.

Water will vaporize at a much more rapid rate the warmer it is, so if not 180°F, then 150°F for a slightly longer period of time. And say, 65°F for an even longer period of time.

Warm sidewalk doesn't have to be 180°F to evaporate water quickly.

But then you have water vapor in the air. It has to get moved out the breather or it remains in the engine to condense again, which brings up a new question for me:

Is the crankcase constantly undergoing added postitive pressure from combustion blow-by, or...whatever? Is it to be expected that air is constantly exiting through the breather?

My thinking regarding operating temperature is: below a certain oil temperature water accumulates faster than it can evaporate, and that more and more water is collected in the oil the longer the engine operates in that condition. That's why many vehicle manufactures consider excessive idling, especially in cold temperatures, to be "extreme" operating conditions requiring more frequent oil changes.

Somewhere along the line I got the idea that the magic number for not collecting water was 180 degrees, though now I don't remember where I got it from.

As for the breather tube...I think air does continually exit the breather, but not at much volume. Most of the moisture has to go out the exhaust with the rest of the combustion byproducts, which is why it's particularly bad for excessive moisture to collect in the oil since getting it out of the engine is problematic.

Or not...I'm no engine mechanic and never will be, but that's my interpretation.

I thought I'd share these pic's of cylinders I removed from my O470K back in Feb,2009. They are oversized steel with about 500 hrs since they were rebuilt in 1995. They have been sitting on a wood deck car trailer in my 50X82 dirt floor machine shed hangar that is a pole building with metal siding. It is uninsulated and goes from well below zero to over a hundred and drips often. I have four(just three in the picture) and two are rust free,one a little rust, and one has quite a bit in places but quite a bit of good. Same environment with big differences so makes it quite a puzzle. They one in the middle is really rusty.

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dogpilot wrote:I may have mentioned in other posts this thing about fuel. Modern fuels, in the attempt to eliminate or lower tetra ethel lead as an anti-knock additive and raise the octane level, added other stuff. So lead was not being spewed all over the place, splendid. What does happen, the blow by component of your exhaust gasses mixes with your oil. Your oil has a dispersant quality, keeps most of this crud in suspension that is too small to filter out. This was fairly easy with lead. The new stuff, not so much so. In fact it raises the pH of your oil. So frequent oil changes, especially if your going to put your aircraft to sleep for the season would be a good start. If you don't fly frequently, have a calendar schedule for changing it, if you fly a lot change it by your hours flown. It may keep your engine from corroding internally.

I often try to push people to explain why they think auto fuel, even without ethanol, would be any worse for an engine than 100LL. It bugs me that people will just parrot what someone else told them without understanding the details. I have always been focused on the fuel system issues that ethanol causes, and of course without the stuff, what could be wrong with auto fuel? It's lead-free, cheaper, and provides enough octane for about 9:1 safely.

Today I started thinking about this aspect of it: the gases that get mixed into the oil. What's in there that's different from 100LL that your engine internals are exposed to? I honestly don't know what's formulated into non-ethanol pump gas, but I do know that 100LL is generally a cleaner fuel. What other shit is added to auto fuel whose post-combustion byproducts could become a corrosive in the case?

Or is it better? I honestly don't know. Someone must have explored this side of it.

Anyone?

Texas goes around and spot checks/pulls samples from service stations. I was told one time they found a tar like molecule that really didn't burn----just passed through. It was about 1% of the gallon and a good way to get rid of part of a barrel of crude and nobody would know. Maybe that is why Texas spot checks.

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WOW what a very informative discussion. Thank you all very much.
I have been interested in aircraft engine issues for many decades and very much enjoy a good discussion on operation and preservation of these most expensive pieces of machinery.

One thing I did a few years back was build a closed loop dehydrator. It was used on a C-90 and an 0300 with similar results.
This was only a very short time test but initial reading were very impressive.
Briefly, got to fly out for breakfast soon, I discovered you only need a small airflow and in a damp hanger the humidity of the air coming out of the engine was usually < 15%.
Many many more factors need to be considered in this rather crude test, but I was encouraged.

Tom

180Marty wrote:Texas goes around and spot checks/pulls samples from service stations. I was told one time they found a tar like molecule that really didn't burn----just passed through. It was about 1% of the gallon and a good way to get rid of part of a barrel of crude and nobody would know. Maybe that is why Texas spot checks.

The last time I used mogas was in my 170 a couple of years ago and I spilled a bit on the wing. It left such a nasty brown residue on the top of the wing I refused to use it again. I wonder if it was this "tar like" substance. It seemed to burn ok but I couldn't get past the thought that if it left that crap on the top of my wing that it must be leaving it in the fuel system too. FWIW it was available at Perryville MO, KPCD, right beside the 100LL pump.

Back to the original post re: engine corrosion. I've started taking the oil cap off after flying to allow the water vapor to escape. I can see visible moisture rising up out of the engine. I don't know if it helps but I figure it can't hurt. I also try to fly at least once a week. And I'm becoming convinced that more frequent oil changes will help get the acids out of the engine.

Very interesting read.

Automotive engines usually have the oil breather plumbed back into the intake. The intake vacuum pulls crap out of the breather and burns it. Is there a reason aero engines don't do this?

A few things to clear up and help keep the discussion on track. The additive in aero shell 100w plus is LW-16702, an anti scuff additive. I have seen nothing from it that claims it stops corrosion. As for camguard, look at VAF at the results Dan Horton got when he made test samples that were coated in camguard, 100w plus, etc.; no discernible difference between any of them.

Brady- take a look at anti splat aero. They make a crankcase breather kit that I’ve had great luck with on my rv-6. It has a relief valve in case the PCV were to get coked up. The marked difference is this system pulls a vacuum on the crankcase by using the exhaust. The side benefit being that the hot exhaust burns up any vapors and it keeps the belly clean.

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