Corrosion 1 week after top OH?

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stevenclin

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Steve Lin
Hi all,

Long time lurker, very occasional poster here. Soliciting opinions on the following issue.

I had a top overhaul done on my IO-550 engine, with all 6 cylinders repaired and rehoned, at a relatively well-regarded engine shop (one of the shops mentioned in a positive light in this Aviation Consumer article).

After picking up the plane, I flew the first 10 hours at low altitude, high power, in three flights during the first week.

Nine days after picking up the plane - having made those three multi-hour flights in the past week - I did the first oil change and borescoped the cylinders.

To my great surprise, there was a fair amount of corrosion on the cylinder walls. Here's an example:

https://drive.google.com/file/d/19hm9ipmGqhnReHflyFOjLCRHDHnnFawb/view?usp=sharing

I called the engine shop, and they told me two, somewhat contradictory, things:

1) What I'm seeing is just surface corrosion, not surprising, even after only a week since top OH, and nothing to be worried about.
2) Run the plane for another 25 hours and it may look better; i.e. with a bit more time, the cylinder walls will get conditioned and some of the corrosion may scrape off.

So, it's now about 2 months and 25 hours later. I have flown 1-2 times per week, mostly long-ish (hour plus) flights, except for one 2-week period when the plane was in annual.

Did another borescope inspection, and it looks pretty much the same; lots of corrosion still. Here's another example pic (different cylinder than the previous picture, but multiple cylinders look roughly like this):

https://drive.google.com/file/d/1ap6tMoe7hIB9IW8Hki_Om-qunm19EJLc/view?usp=sharing

So, I guess my question is: is it normal and expected to see corrosion like this a week (or even couple months) after a top overhaul with rehoned cylinders, having flowed regularly throughout that period? Or, am I getting the runaround from the engine shop after they did a poor quality top overhaul?

My main concern is that with so much corrosion immediately after the top OH, these rehoned cylinders will not have a long life ahead of them before another overhaul is necessary. And, if the corrosion indicates a poor overhaul, I'd like to get it addressed ASAP, rather than trying - months or years from now if an early overhaul is indicated - to bring it back up with the engine shop then, who will (rightly) likely ignore me then.

Thanks for any advice & opinions!
 
The cylinder walls look really smooth with no crosshatch in the corrosion area. There are some smart engine guys here that know better than I do but I wouldn’t be really excited about that in my engine.
 
Pretty much every aircraft engine I’ve looked in that doesn’t have plated cylinders on it will have some light pitting/corrosion sooner rather than later.

Borescopes can be useful tools, but they can also cause unnecessary alarm. It’s hard to tell from the picture but is it possible you’re seeing a reflection from some irregularity on the piston or cylinder head?

I work in an engine test lab and have seen several instances where an engineer will request a tear down inspection after seeing something on a borescope image that turns out to be nothing once things are apart.

Im not going to say anything is good or bad here but if it bothers you I’d suggest finding a good local engine guy and have them take a look at the engine in person rather than relying on pictures and opinions based on them.
 
It looks more like coked on oil. The second pic with the line through the dark spot looks suspiciously where a piston ring was resting. The black portion also appears proud of the surface.

We’re the pistons replaced and/or re-ringed? The verticals striations look suspicious. The second pic also shows signs of scuffing, especially on left side.
 
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Yeah, that looks like crap. I'd think it was corrosion too and I would suspect that the cylinders went in like that, plus the shop never suspected the cylinders would get borescoped 25 hours after install. You are right to ask questions and push the issue.
 
May not be relevant, but a former employer spent a lot of money at the prop shop at the place in NC mentioned in the article (on a problematic constant speed unit), and it came out with the same problem it went in with.
 
Also unsure of relevance, but let's not forget what I posted some time back about an acquaintance's valves and the limitations of new technology ("borescoping has limitations and can produce false positives").

Have a look at these exhaust valves from two cylinders; each cylinder with a pic taken Jul4, and then again Jul10.
The engine sat unused between the photos while undergoing a condition inspection, the engine was not started; the plugs were left out between photos.

Some say "unbelievable!", many 'experts' remain baffled.
My conclusion is that it is lighting/reflection/camera angle.

Cyl3 Jul4:
Cyl3 Jul4.jpg


Cyl4 Jul4:
Cyl4 Jul4.jpg

Cyl3 Jul10:
Cyl3 Jul10.jpg

Cyl4 Jul10
Cyl4 Jul10.jpg
 
Right after honing there are many nooks and crannies where corrosion can get a foot hold. If the engine is running well and not using a crazy amount of oil for this stage of break in put the bore scope back in the toolbox and fly the airplane. The best prescription is to fly it regularly.

I think you are falling into a mode of “analysis paralysis.”
 
It doesn’t look like corrosion to me. New cylinder corrosion looks like a haze of rust colored rust. If oil consumption and compression are good I’d continue with the break-in.
 
Honing the cylinder won't remove the deeper pitting, it just covers it kinda. I had a used cylinder honed by an engine shop looked fine at the time, a week later with it sitting in the box and the pitted areas turned red making it obvious that it wasn't new cylinder and never could impersonate one.
 
The most corrosion-vulnerable cylinders are newly honed cylinders. The old rule I was taught is it takes 100 hours to get them seasoned.
 
NordicDave said:
It looks more like coked on oil. The second pic with the line through the dark spot looks suspiciously where a piston ring was resting. The black portion also appears proud of the surface.

We’re the pistons replaced and/or re-ringed? The verticals striations look suspensions. The second pic also shows signs of scuffing, especially on left side.
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Stewartb said:
It doesn’t look like corrosion to me. New cylinder corrosion looks like a haze of rust colored rust.
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What other maintenance was done to the cylinders?:
  • Were new pistons installed?
  • What was the condition of the piston skirt if piston reused? Was it measured to be within service limits?
  • Were new rings installed & gapped, or were the old rings reused?
 
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Where with respect to the engine axis are these marks happening? Can you get a finger through a sparkplug hole and feel if there is any detectable corrosion? My guess it you won't feel any.

It is almost like it didn't clean up from honing except the clear, which is obviously from a ring area, in the second picture. See post 12 above.....
 
If that is corrosion it's not new. When you hone cylinders there's only so far you can go and you want to remove as little material as possible. I'm thinking the only option the shop could have given you at overhaul would have been to reject the cylinders and have you buy new ones. Would you have been okay with that? Regardless, what you are seeing is not something that happened over the past few weeks or is developing at a rapid rate and you got a top overhaul, not a new engine.
 
OP here - thanks for the responses, I appreciate it. Sounds like there are a variety of opinions (not too surprising!). To answer some questions:
  • The rings were replaced with oversize rings (P010). Original pistons. I don't know about the piston skirt.
  • At the time I brought in the engine, I told them I'd prefer to just repair the existing cylinders, but I'd be open to replacing with new if necessary, so I was not pressuring them to repair these over replacement.
  • The corrosion (or non-corrosion, whatever marks) are toward the bottom of the cylinder - at least, as far as you can get the scope in. In other words, turning the prop till the piston is bottomed out, the discoloration is not too far above the piston. Don't think my fingers are long enough to be able to reach most of it.
  • Compressions are all in the 70s, but they were in the 60s and 70s before the top OH as well. The primary symptom before the OH was excessive oil consumption, with roughly the same amount of oil pooling in all the cylinders, which is why I opted for the top. The oil consumption post OH is much better.
Thanks again for the comments, and I'm open to more if anyone has any additional thoughts. Although a couple of you seemed to think the same as me (there's a problem here), seems like the majority feel I'm probably over-reacting. So I'll mull it over a bit, I guess. In any case, thanks again.
 
It would be nice if you could determine if those are deposits or depressions below the cylinder surface due to corrosion. You said the rings were oversize, by the number you posted I assume is 0.010 in over. That's quite a bit of material that needed to be removed from the cylinder for those to fit. I'm a little surprised the pistons weren't replaced too, but honestly I don't know enough about airplane engines to tell you this is right or wrong. I would seek out a couple mechanics an ask them to look at the pics for an opinion.
 
stevenclin said:
The rings were replaced with oversize rings (P010). Original pistons. I don't know about the piston skirt.
Click to expand...

I'd be doing some homework here. You don't use oversize rings on standard pistons. Either these cylinders have been bored sometime in the past and already had oversize pistons that required oversize rings or something is wrong.

As others have pointed out, there could have been pitting there before the work was performed to the cylinder. Honing likely wouldn't take the pitting out, or at least not without making the bore so large that oversized components would be needed (the cylinder would likely be rejected at that point). If it is pitting, it being farther down the bore won't affect compression measurements because that is performed with the piston at TDC. The worst you'll see out of this is the potential for a slightly higher oil consumption rate than what you'd have with clean cylinders but even then it will likely not be a measurable amount more.
 
mondtster said:
I'd be doing some homework here. You don't use oversize rings on standard pistons.
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You will if the honing procedure has removed enough material. Of course there is a limit to this but you will use oversized rings if the end gaps on standard rings go out of spec. You then need to file the ends of the oversize rings to fit. Yes, you are right on the edge of where you would need to bore oversize but not quite there so this method works.
 
Silvaire said:
You will if the honing procedure has removed enough material. Of course there is a limit to this but you will use oversized rings if the end gaps on standard rings go out of spec. You then need to file the ends of the oversize rings to fit. Yes, you are right on the edge of where you would need to bore oversize but not quite there so this method works.
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On standard size pistons? Not oversized?

How do you plan resolve the resulting piston to bore clearance problem that would most certainly result?

I’d much rather have loose ring clearances than try what you suggest.
 
mondtster said:
On standard size pistons? Not oversized?
How do you plan resolve the resulting piston to bore clearance problem that would most certainly result?
I’d much rather have loose ring clearances than try what you suggest.
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Well I'm not suggesting anything, just pointing out that this is indeed done and thus explaining what the OP reported. You seem to be assuming that they are putting standard pistons in a 0.010 over cylinder but this is not the case, it may be 0.003 or possibly 0.005 over at the extreme end after honing. As far as a decision to go with out of spec ring gaps, that would be up to you, if that's what you'd rather do then by all means but I'd at least do some research and talk to an experienced rebuilder about it. There's more than one way to skin a cat.
 
Silvaire said:
Well I'm not suggesting anything, just pointing out that this is indeed done and thus explaining what the OP reported. You seem to be assuming that they are putting standard pistons in a 0.010 over cylinder but this is not the case, it may be 0.003 or possibly 0.005 over at the extreme end after honing. As far as a decision to go with out of spec ring gaps, that would be up to you, if that's what you'd rather do then by all means but I'd at least do some research and talk to an experienced rebuilder about it. There's more than one way to skin a cat.
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I’m not assuming anything. I’m suggesting that investigation needs to be done if parts are mixed and matched the way the post reads.

I wouldn’t put stuff together the way you’re suggesting but I have no doubt some might try. It doesn’t make it any more desirable or acceptable in my book.
 
OP here one more time. As should be obvious by my questions, I'm not an engine guru, and I did not realize it was not possible (or at least controversial) to reuse a piston with oversized rings. All I know is that on the invoice I received, the parts called out 1 piston and 6 oversized ring sets. So, my interpretation/understanding was that one piston had to be replaced (meaning the other 5 are original), although it could have been a typo on the invoice. I didn't mention the "1 new piston" above, since 5/6 of the pistons were, to my knowledge, the original ones, I'm seeing the corrosion or discoloration on multiple cylinders - so it's clearly not related to the "1 piston" that was replaced - and I didn't want to get too lost in details. But putting it out there now for additional clarification! :) Thanks again to all for the comments/suggestions.
 
Reusing pistons is fine. What is unknown at this point is what the pistons which were reused are. You might have to go back in the logs and see if you can determine what was previously installed. They might be oversized, in which case the oversized rings match the oversized pistons. What is the part number of the piston they replaced? Is it standard or oversized?

Cylinders often come with paint markings on them to signify what they are. For example, orange is chrome, blue is nitrided, etc. If there is a yellow or green stripe it would suggest the bore may be oversized. Location of the marking is also important as it might mean something different.

Anyway, these questions are things that might best be answered by the mechanic that did the work, not a bunch of us on the internet providing responses with very little to go on.
 
mondtster said:
...I wouldn’t put stuff together the way you’re suggesting...
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Again, I'm not suggesting anything to anybody. I'm just pointing out why the OP might have what he's got. People should understand when they pay for a top, or even major overhaul it doesn't necessarily mean they're getting new pistons and cylinders and there's a wide spectrum of what an overhaul can be. In general parts are going to be replaced when worn beyond limits and reused if they're not. That may not be the way you would do it and that's fine but believe me it's been done all kinds of ways for years and cutting down oversized rings for a honed out cylinder is one of the things that gets done, there's nothing wrong with it. I can't, based on what we've seen, say from my comfortable chair here what the absolute status of the OP's engine is but there's at least a good chance that he's going to get many, many happy hours out of it.
 
Silvaire said:
Again, I'm not suggesting anything to anybody. I'm just pointing out why the OP might have what he's got. People should understand when they pay for a top, or even major overhaul it doesn't necessarily mean they're getting new pistons and cylinders and there's a wide spectrum of what an overhaul can be. In general parts are going to be replaced when worn beyond limits and reused if they're not. That may not be the way you would do it and that's fine but believe me it's been done all kinds of ways for years and cutting down oversized rings for a honed out cylinder is one of the things that gets done, there's nothing wrong with it. I can't, based on what we've seen, say from my comfortable chair here what the absolute status of the OP's engine is but there's at least a good chance that he's going to get many, many happy hours out of it.
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Nobody can. We're saying the same thing but apparently you interpreted what I wrote differently than intended.
 
That looks like corrosion that was too deep to remove without going beyond the service limits. If it was carbon the hone would have taken it out. I think those cylinders were merely roughed-up so the new rings would seat, with very little metal removed. The new piston-to-cylinder clearance is .008" to .011, and service limit is .014". If that engine started out with a clearance of .011" and the the clearance increased maybe .002" due to wear, you're already within .001" of the limit. You can't go honing aggressively. It's a matter of honing just enough to get the surface roughness required for ring seating.
 
Dan Thomas said:
That looks like corrosion that was too deep to remove without going beyond the service limits. If it was carbon the hone would have taken it out. I think those cylinders were merely roughed-up so the new rings would seat, with very little metal removed. The new piston-to-cylinder clearance is .008" to .011, and service limit is .014". If that engine started out with a clearance of .011" and the the clearance increased maybe .002" due to wear, you're already within .001" of the limit. You can't go honing aggressively. It's a matter of honing just enough to get the surface roughness required for ring seating.
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:yeahthat: This is where I was leading with my earlier post number 12 about questions concerning clearances.

Anyone installing a cylinder for the first time may not completely appreciate the partnership between their mechanic and the cylinder shop. A critical step in the process is mutual understanding of what’s required ensuring a proper hone pattern, service limit tolerances and proper ring gap. Unless the piston is also sent to the cylinder shop, it’s really up to the mechanic to measure and make a few critical decisions. The cylinder shop’s work may be based on decisions/observations from the MX and likewise the returning cylinder needs to be carefully inspected by the mechanic as the final step on the preinstallation quality chain.
 
Dan Thomas said:
That looks like corrosion that was too deep to remove without going beyond the service limits. If it was carbon the hone would have taken it out. I think those cylinders were merely roughed-up so the new rings would seat, with very little metal removed. The new piston-to-cylinder clearance is .008" to .011, and service limit is .014". If that engine started out with a clearance of .011" and the the clearance increased maybe .002" due to wear, you're already within .001" of the limit. You can't go honing aggressively. It's a matter of honing just enough to get the surface roughness required for ring seating.
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That was my first impression as well.
 
I oil consumption is normal and also compression 70 or above I would have little safety concern. Just fly. As one poster mentioned Milleniem cylinder assemblies are so reasonable many just replace the cylinders. And you get new pistons. valves, etc.
 
Honing can’t remove all the pitted areas. If you want it to look new you need new. I don’t like it either, but it is a repair, not new, or even a overhaul. If it’s making good numbers I’d probably just keep a close check on it. I suspect it’s only going to get better with continued use.

I’m struggling with a similar situation on a IO 520. Trying to decide on a top OH due to someone repairing instead of replacing previously. It’s on a plane that I’m in the process of purchasing. During the previous zero time overhaul they did not go on with new cylinders! They only “repaired” the cylinders. I didn’t realize shops were even doing this, and sill calling it zero SMOH! I guess that’s what I get for being on the right side of this business!
 
Well you chose not to buy new cylinders...
Like others have said here. That is not surface corrosion. That is deeper and will not go away because your rings are not even touching that area.
Does it have good compression? The last overhaul we did brand new cylinders were purchased because there really wasn't much price difference.
I don't understand why anyone would overhaul a high time cylinder on a 550?
 
If it's at the bottom of the cylinder I don't think you're ever going to notice it. By the time the piston gets that far down the exhaust valve is already opening so you're not going to get much blow-by, if any, and the corrosion is relief so it's not wearing the rings. I'd even go so far as to say it's just cosmetic and of no real effect.

There, feel better now?
 
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