Gauging risk on an old engine

[ultrarunner]

Hypothetically speaking, say we have a common engine, a Lycoming O-320, with 400 hours SMOH. Past infant mortality, and running well.

Then we let it sit for 10 years.

Differential compressions are still >75, and no metal is found upon changing the oil. What are the likely failure modes for this engine? Short term, will cylinder wall rust be caught by the filter (assuming it has one)? Will increased cam lobe wear (or anything else) result in a sudden stoppage? Or are we simply anticipating a much shorter service life (<200 additional hours) until performance is no longer acceptable?

 

[Mikey52]

It is my understanding but I'm no expert, that if pitting on the cam is bad enough when the engine is put back in service. It could be less than 10 hours to when lobe wear is bad enough to be unacceptable.

https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-09_cam-distress.pdf

 

[Jim K]

Most likely case is the cam/ lifters have developed some rust. The engine will run fine for.... maybe 20 hours, then gradually lose power as the cam lobes wear to round. The good news is it won't be a catastrophic failure and will give plenty of warning. Change the oil every 20 hours and metal in the filter will let you know if it's coming apart. Don't fly it out of short strips or over hostile terrain.

If it makes it 100 hours, it'll probably give close to normal service. If it was stored in a dry climate, it's possible you could get away with it. Anywhere near an ocean or the midwest... budget for a new engine.

 

[RyanB]

I will give you my two cents based on my experience with a very similar situation.

The engine will likely be OK for a little while, but once it starts flying routinely again, those gremlins from sitting idle for long periods begin to appear. I would go on a limb and say it probably will not make it beyond a few hundred hours without some sort of cylinder work or possibly a rebuild. That said, I would expect at minimum in the near term, it needs new hoses and accessories. Of course it’s highly possible it could last, but letting it sit that long just kills these engines.

 

[brcase]

Most likely case is the cam/ lifters have developed some rust. The engine will run fine for.... maybe 20 hours, then gradually lose power as the cam lobes wear to round. The good news is it won't be a catastrophic failure and will give plenty of warning. Change the oil every 20 hours and metal in the filter will let you know if it's coming apart. Don't fly it out of short strips or over hostile terrain.

If it makes it 100 hours, it'll probably give close to normal service. If it was stored in a dry climate, it's possible you could get away with it. Anywhere near an ocean or the midwest... budget for a new engine.

Friend of mine recently went through this with an O-320. 1st for symptom of the cam problem was a noticeable loss of power. Taking off from an airstrip he was pretty familiar with suddenly became a scary experience as he barely cleared the trees at the end, which was NOT normal.

But then his entirely new engine (upgraded to something like a io-390, but I don't know which one for sure) he installed did the same thing within 50hrs, but that was likely covered by warranty. I think they caught this one at the oil change and the engine making metal.

Brian

 

[TCABM]

O-320s have a 2000hr or 12 yr TBO, recommendation, whichever comes first. Since it’s unlikely to put 1600hrs on it in two years, I’d gauge the motor as having 83% of it’s TBO time used. Put differently, there’s 17% of time left until TBO.

Operationally, I wouldn’t want to be PIC for the first the 20 hours of resurrection on it and after that I’d want oil analysis results at a minimum before thinking about flying it.

Call me conservative.

 

[MRC01]

Most likely case is the cam/ lifters have developed some rust. The engine will run fine for.... maybe 20 hours, then gradually lose power as the cam lobes wear to round.
...
If it makes it 100 hours, it'll probably give close to normal service. If it was stored in a dry climate, it's possible you could get away with it. Anywhere near an ocean or the midwest... budget for a new engine.

This is similar to my experience buying a C-172 with a O-320-D2G engine in 2008. All tests said it was healthy at about 850 hours SMOH, though we didn't inspect the cams because doing so is too invasive. I got 9 years / 900 hours of out it, when one cylinder had low compression. Burned exhaust valve. We pulled the cylinder and saw the cam for the first time. Early stages of light corrosion and spalling had just started. According to the Lycoming guidelines it was marginally airworthy. But since it was nearing TBO and also had a bad cylinder, I decided to replace that engine.

The lesson: Lycoming cams are prone to corrosion. Periods of unuse increase this risk but some risk is always there no matter what, as mine never sat more than a week or two between flights. Mine didn't show elevated metals in UOA nor metal in the oil filters. That could be because the corrosion was in the early stages. The way these engines are designed, you have to split the case to replace the cam, so at that point you might as well overhaul or replace the engine.

 

[frfly172]

If the motor was not pickled properly I would be Leary of trusting that motor,just me.

 

[Tusayan]

I fly an O-320 that was assembled in 1971 and has never been apart. It has 1150 hrs total time with a known history that includes some significant periods of storage. I’ve been flying it 300 hrs and oil consumption has risen from 1 qt every 12 hrs to the same every 8-9 hrs. Aircraft performance is fine. There’s some corrosion on 3 of 4 cylinder walls but compression is fine and there’s no metal in the filter. Oil analysis has not been done regularly, but when done has been fine.

I doubt based on my experience that anything dramatic will happen when the engine is flown. It might but I doubt it. Watch it carefully and it will tell you when it’s ready to be disassembled. In a circumstance like this, manufacturer TBO numbers are not terribly relevant.

 

[Dan Thomas]

Speaking from experience: if that engine has sat for those ten years without being started and run, it might be just fine inside. Magman tells the story of one airplane, Lycoming-powered, that sat on jacks in a hangar to keep it dry since the area flooded periodically. It sat there like that for years. Then the engine was taken off and opened up, and everything in it was perfect. No corrosion.

Atmospheric moisture is a tiny problem compared to the blowby moisture that gets into that crankcase when the owner starts and runs it, thinking that he's doing it some good. He's wrecking it. I have seen it too many times in airplanes I've worked on. These thigs need to fly, or be left strictly alone. Storage oil will help.

 

[Gary Ward]

I got lucky here in humid Cincinnati Ohio. When I started flying friends plane it sat mostly for 10 years prior showing 9 hrs flight time during those prior 10 years. I think some of the time aircraft sat outside. And it had the dreaded 0-320 H2AD engine in it.
Turned out to be just fine. It had 1400 hrs off of it and I flew it 500 more. It never made metal and I even removed cylinders toward the end and the cam and lifters looked fine. The lifters did show some corrosion but no spalling. The lobes had microscopic pits but not enough to start spalling. Only could see the pits with magnification.

I'd do what others said fly it and and keep an eye on the filter and how it's running. I would take a gamble on it...

 

[ateamer]

Speaking from experience: if that engine has sat for those ten years without being started and run, it might be just fine inside. Magman tells the story of one airplane, Lycoming-powered, that sat on jacks in a hangar to keep it dry since the area flooded periodically. It sat there like that for years. Then the engine was taken off and opened up, and everything in it was perfect. No corrosion.

Atmospheric moisture is a tiny problem compared to the blowby moisture that gets into that crankcase when the owner starts and runs it, thinking that he's doing it some good. He's wrecking it. I have seen it too many times in airplanes I've worked on. These thigs need to fly, or be left strictly alone. Storage oil will help.

I contacted the seller of a plane that had 0 SMOH on a Lycoming. The ad didn’t say when the overhaul was done. My first question was that. He said it was two years ago, but he starts it and does runups on the ramp.

Ugh. Conversation terminated.

 

[Magman]

Ground Run for 10-20 HOURS?

I think you’ll cook the heads before oil temp becomes excessive.

No thank you!

 

[Gary Ward]

I would run it >2000 RPM on the ground for 10-20 hours before I took it airborne. Then maybe a bunch of straight ahead takeoff/landing cycles on a long runway to really stress test it followed by a full Blackstone analysis and very careful filter analysis. A borescope would be good as well. These are all very easy and inexpensive precautionary steps before you risk your life on it. Better safe than sorry.

I don't think that would be best?
I took off for the first flight from a drag strip with a unknown aircraft and engine. I flew a pattern above the strip for an hour or so in case I needed to land. The owner was right there swearing the plane was good to go.
Hour later I ventured a whole 5 miles to a nearby airport that he rented a hangar at for his plane so I didn't have fly it out of the tricky dragstrip. Little nervous during that 5 miles over hills covered in trees. Then I flew for another hour or so above that airport. Landed with 2 hours under my belt and proceeded to flying just about everyday. It built up my confidence in it pretty fast.

 

[Dan Thomas]

Ground Run for 10-20 HOURS?

I think you’ll cook the heads before oil temp becomes excessive.

No thank you!

Exactly. The CHTs will run away while the colder crankcase is busy collecting water. Good way to ruin it one way or the other.

 

[Gary Ward]

I am thinking he meant 10-20 minutes? before taking off for the first time?

 

[Magman]

A re-read of OP’s post strikes me as he would like input on the likelihood

engine failure in flight vs eventually needing an o/h.

My thought is the former is unlikely if properly inspected and monitored

at closer intervals than normal. A check of Static RPM on every TO is key.

 

[Bell206]

What are the likely failure modes for this engine?

Depends on too many variables to make an accurate call. I've seen similar engines last till when an OH was needed and at the same time had one basically come apart on the 1st ground run. Are there some actions/tasks you can perform that will mitigate some of the possibilities? Sure. But again most to be provide an accurate assessment of the engine's internal condition/health are usually subjective to the specific engine.

So in a hypothetical sense, to give a better answer it would depend on what your main concerns are and whether this engine is in your possession or not.

 

[ultrarunner]

A re-read of OP’s post strikes me as he would like input on the likelihood

engine failure in flight vs eventually needing an o/h.

My thought is the former is unlikely if properly inspected and monitored

at closer intervals than normal. A check of Static RPM on every TO is key.

This is indeed what I'm asking. An overhaul is guaranteed. Safe extrication from a hostile airport, and the reasonable radius of places to bring it is the question right now.

More information: it was hangared, and not ground run as the owner was no longer present. The plain steel cylinders had some corrosion upon borescope, but it appeared to be largely surface rust. I soaked the cylinders in marvel mystery oil for a while before spinning the engine by hand until we were able to verify oil pressure. Did an hour of aborted takeoff runs making good RPM and monitoring CHTs. Changed the oil & cut the filter, with no metal found. Compressions 75-78. Checked sump screen & carb bowl for debris and found none. It does have an air/oil separator so I'm hoping the crankcase was sealed off from the elements but that's just a hope.

I'm not local to the plane and it's just had its annual signed off, so the plan is to orbit the field 3000-4000 AGl for an hour or two before hopefully heading to a new shop. How it performs throughout this process will determine next steps. At this point, attempting to make plans, I do want to verify my understanding that catastrophic sudden failure is relatively low.

 

[kyleb]

Pull a mag and look at the gears in the accessory case. That’s a reasonable stand-in for the cam and lifters. If the gears aren’t rusted/pitted, that’s a good sign.

 

[Clip4]

Really there is no way to tell. I had a friend who was flying behind a 30 year old engine with less than 500 hours when he bought it, he flew it a lot, and the same engine was running fine when he sold it.

 

[Bell206]

More information:

This is the type of info needed to offer any input of value. Appears you've started on the right path. What I would also recommend is to review the fuel cells and delivery system a bit more vs just at the screen/bowl. Also check to make sure the battery is charging properly. And before heading to your new shop perhaps fly the pattern for 10-15 minutes then land and shutdown. After a good review of the aircraft for leaks etc, then do your orbits and flight to new shop. I've found if there is going to be an obvious system failure/issue it tends to happen in the 1st 10-15 minutes then the last 10-15 minutes. For reference, a number of commercial ops also use a similar technique to access things after certain maintenance tasks and it provides a good quick check of things. Good luck.

 

[ultrarunner]

I really appreciate everyone's responses. We'll keep moving forward with the plans and report back after weather aligns.

 

[ultrarunner]

Well, we moved the plane.

I wanted to write this up for posterity if anyone else attempts something like this.

The first thing I noticed was that it has a consistent stumble when opening the throttle. Possibly a dried up accelerator pump diaphragm? It recovers quickly and it's smooth when operated slowly, I just did that. Several full power static runups and aborted takeoff runs were completed with no signs of power loss or roughness, so a go decision was made. Even with inspections and tests, taking off above the alligator infested lake near the airport was one of my braver moments.

We got a nice backfire (afterfire) upon pulling back the throttle at top of climb. Interestingly, this did not repeat itself for the rest of the trip, but research pulled up discussion of the "Grumman Stumble". I do have the 10-5217 carb with pepperbox main. Maybe it's plausible that some of the holes were clogged with old avgas. Other than this, performance was generally what I expected, with climb rates >500fpm and cruise around 110 KIAS.

I suspect the FS450 reads way high, as it indicated around 12gph leaned in cruise at 4500 and 10gph at 8500. This, with the EGTs around 1500-1550. Closing the throttle at all would cause EGTs on cylinders 1 and 2 to drop to 1300 and cylinders 3 and 4 to climb toward 1600, so I tried to avoid that. It would still run smoothly, though, and CHTs were never a problem, staying around 400 in initial climb and dropping to 360-380 in cruise.

At one point climbing out of 8000, we got a major stumble and a series of bangs out of the exhaust. I had the fuel pump on, switched tanks, and mixture in before the other part of my brain asked what was going on. I decided that I was above an airport (yay for planning) and it'd be foolish to try to jump to the next one. This would turn out to be a financial mistake, but a good move nonetheless. They found 4 spark plugs reading in the 300k—1MΩ range (two of them were on cylinder one). The gaps were also all too wide. I have a log book entry for cleaning, gapping, and testing the plugs from 5 hours previous, but maybe given their age I feel hesitant to start pointing fingers.

We replaced 4 plugs with overnighted new ones, and I removed the carb fuel inlet screen again to clean it out, but there wasn't much in there. It ran noticeably smoother after this, but the partial throttle EGT disparity between 1+2 and 3+4 remained. It developed an oil leak behind the newly replaced vacuum pump, so it's hard to nail down numbers, but we consumed 1 quart the first hour decreasing to 1 quart over the last four hours.

It's now stored safely while we wait for time on the schedule. I ran out of time away from work so I've asked them to cut the filter so we can make some choices. After talking with the shop owner, I'm leaning toward an IRAN (gaskets & cam) and top with high compression pistons. At least if we pull a cylinder we can make a call on the cam and splitting the case.

So that's it. The theories that O-320s generally seem to fail over the long term through loss of performance rather than in spectacular fashion seem sound. On an old engine, I'm way more concerned about fuel issues than metal breaking.

 

[Gary Ward]

If you were paying for overnight shipping why not replace all the plugs?
Oil leak from behind the vacuum pump means you need another new vacuum pump along with a seal.
I would cut the filter myself and if it looks ok keep flying it. It sounds like it is too soon to start pulling cylinders off of it?

 

[Dan Thomas]

The first thing I noticed was that it has a consistent stumble when opening the throttle. Possibly a dried up accelerator pump diaphragm? It recovers quickly and it's smooth when operated slowly, I just did that.

Weak spark can do that. Might be mags, leads, or plugs, or all three.

The accelerator pump has a piston with a cup seal, not a diaphragm. There are tiny check valve balls that might be stuck due to varnish from dried-up fuel.

We got a nice backfire (afterfire) upon pulling back the throttle at top of climb. Interestingly, this did not repeat itself for the rest of the trip, but research pulled up discussion of the "Grumman Stumble".

Weak spark, again. The mixture will tend to lean as the throttle is closed a bit.

They found 4 spark plugs reading in the 300k—1MΩ range (two of them were on cylinder one). The gaps were also all too wide.

Well, there we are. Old Champions, right? Bad resistors combined with inept gapping.

but the partial throttle EGT disparity between 1+2 and 3+4 remained.

That is often caused by the fuel spray from the main nozzle striking the throttle plate. That plate will deflect more fuel to the front cylinders (1 & 2), cooling them, and less to the rears, which will get leaner and the temps will climb.

 

[ultrarunner]

Excellent info, Dan, thank you. I was told by the original mechanic that the accelerator pump was a leather diaphragm that shrinks when dry for an extended period and tends not to return to its original shape. That didn't entirely make sense to me, but he's got 50 years of experience and there was enough else going on. Looking at the overhaul manual I can see it's a plunger driven by a leather "spring".

That is often caused by the fuel spray from the main nozzle striking the throttle plate. That plate will deflect more fuel to the front cylinders (1 & 2), cooling them, and less to the rears, which will get leaner and the temps will climb.

Is this due to the nozzle being partially clogged or just inherent to the design? I've been wondering if sending the carb out for overhaul makes sense. We were originally planning on an entire engine overhaul but it's sounding like it may not be time for that yet.

 

[Gary Ward]

When I was training in C172S with fuel injection, if I pulled the throttle back too fast it backfire.

 

[MBDiagMan]

Hypothetically speaking, say we have a common engine, a Lycoming O-320, with 400 hours SMOH. Past infant mortality, and running well.

Then we let it sit for 10 years.

Differential compressions are still >75, and no metal is found upon changing the oil. What are the likely failure modes for this engine? Short term, will cylinder wall rust be caught by the filter (assuming it has one)? Will increased cam lobe wear (or anything else) result in a sudden stoppage? Or are we simply anticipating a much shorter service life (<200 additional hours) until performance is no longer acceptable?

It will take several oil change cycles for the metal to really show up. Been there, done that on a Lycoming.

The only way I would buy that plane is if I could buy it cheap enough to afford a field overhaul and put the new DLC Lifters in it….. OR, if I wanted to personally split the engine, replace the lifters and put rings in it to ensure that they aren’t stuck from age developed sludge. It wouldn’t be near the problem, even with the high cam, if it didn’t have mushroom lifters which make it impossible to replace them without splitting the case. That’s where it turns into a high dollar overhaul as opposed to a top overhaul.

 

[MBDiagMan]

If the motor was not pickled properly I would be Leary of trusting that motor,just me.

How do you “pickle” a Lycoming to prevent lifter corrosion? The only thing I can think of is to pull the cylinders and coat the lifters with cosmolene or some such.

 

[Dan Thomas]

How do you “pickle” a Lycoming to prevent lifter corrosion? The only thing I can think of is to pull the cylinders and coat the lifters with cosmolene or some such.

The oil is drained out and preservation oil put in, and the engine run for a few minutes.

https://www.shell.com/business-cust...centre/technical-talk/techart09-30071425.html

 

[Bell206]

How do you “pickle” a Lycoming to prevent lifter corrosion? The only thing I can think of is to pull the cylinders and coat the lifters with cosmolene or some such.

A special preservation oil is run in the engine for a prescribed time and then the engine is sealed up from the atmosphere. The use of Desiccant packs is also recommended in certain situations. Then there is a regular follow up to monitor the engine. Lycoming has a bulletin on this which when done properly also preserves various warranty requirements.

 

[Gary Ward]

I received my engine off the back of a UPS truck in the rain, worst possible storage conditions I was thinking at the time. lol It sat in my hangar for 75 days until it got put in and started. Didn't move the crank until the day I started it. Preservative oil ran out of every opening on the engine.

 

[MBDiagMan]

I’ve been through the long time stored Lycoming saga and from my experience I wouldn’t put a lot of confidence in such measures. They obviously would help, but the stakes are too high to trust that totally. Only my opinion. I don’t recommend investments to others and I wouldn’t recommend this risk either. Again, only my experienced opinion.

One thing that would make that a possible step for me would be if the engine had DLC lifters.

 

[MBDiagMan]

I received my engine off the back of a UPS truck in the rain, worst possible storage conditions I was thinking at the time. lol It sat in my hangar for 75 days until it got put in and started. Didn't move the crank until the day I started it. Preservative oil ran out of every opening on the engine. Seem like there was more than 8 qts? Was the camshaft submersed in oil do you think?

I expect that such factory packing sealed it pretty well. As long as it didn’t get so soaked that much moisture made it through the packaging, I expect you’re good.

If this is a relatively new Lycoming factory or factory reman, it probably has the DLC lifters.

Edit:

I see on the label an April 2020 date. I don’t know if they were using DLC lifters then or not. I don’t think so. It would be very good to know that. It probably wouldn’t take much research to find out. If it does, that would be a big selling point if you ever store it and then want to sell it.

 

[Gary Ward]

I doubt it does have those lifters, it's 0-320H2AD engine. I covered it drove to the hangar that day. It was dry before I went home that day with a floor fan.
I fly it 3-5 days a week, now has 690 hrs since July 2020. It does not sit much.

 

[Bell206]

I’ve been through the long time stored Lycoming saga and from my experience I wouldn’t put a lot of confidence in such measures. They obviously would help, but the stakes are too high to trust that totally. Only my opinion. I don’t recommend investments to others and I wouldn’t recommend this risk either. Again, only my experienced opinion.

That is not my experience after preserving dozens of Lycomings. However, the error I see most people commit is that the preservation procedure is not "set and forget" process and requires regular monitoring. Without that monitoring then yes you can have issues. Have had preserved engines go years with zero corrosion or issues to include the lifters.

 

[ultrarunner]

For anyone following along, we finally got a cylinder off. To recap, O-320 stored in a large hangar in central Florida. There was a non-aviation business in the same hangar and the door was frequently open. It does have an air/oil separator if that's relevant (my thinking being that there was less interchange between ambient and crankcase air).

Cylinders are in really rough shape, but it looks like they were probably machined that way during the overhaul. Very deep "cross hatching" spirals. There was also some rust blotching. This engine had differential compressions consistently over 78 (one place returned all 80/80, which I don't think is possible). My A&P/IA said it looks like they tried to hone them .010 over with 60-grid sandpaper.

However, the cam & lifters are fine. We're all pretty surprised.

It'll get four new cylinders, a surefly, and we'll go from there. Just another anecdote for the pile.

 

[chemgeek]

FYI, the older Grumman carburetors had a slew of service bulletins regarding the venturi. I hated that carburetor. None of the venturis (I think my carb went through 3 different designs) worked properly. The engine would stumble every time the throttle was retarded at altitude, and would often stumble when throttle was applied for takeoff. When I overhauled the engine, I got a brand-new carb. It works flawlessly.

My original (to me) engine was a field overhaul special, but it made it to 1900 hours with 25 years since its last overhaul. I had to pull a cylinder because it was pumping oil due to advanced wear (not too unexpected for a 1900 hour cylinder), and while it was off discovered the beginnings of spalled lifters and advanced cam wear. That was enough to make the call for a major overhaul. The engine made close to book power and fuel consumption despite the lifters and worn cam. Any power decline due to wear was gradual and not easily noticeable. The new engine, however, was a revelation in power.

I'd recommend using CamGuard in your oil to forestall, as best you can, any further corrosion of the bottom end. That bottom end corrosion is what killed my original engine. (The top end fared really well.) I use CamGuard religiously in my new engine, and expect a long service life.

 

[Dan Thomas]

It does have an air/oil separator if that's relevant (my thinking being that there was less interchange between ambient and crankcase air).

That air/oil separator also catches and returns water to the crankcase. Not wise at all.