How much stock do you put into a top overhaul?

[ArrowFlyer86]

I'm seeing more planes over the last 6-12 months that are putting top overhaul times down in lieu of a SMOH. Sometimes they're upfront about it, other times they'll kind of bury it in the details of the description.

My question:
As a buyer, how much faith do you put in these TOH in terms of extending engine longevity? Do you give sellers any "credit" so-to-speak for having had one done?

I've always kind of thought of it as more of a red flag, but I'd like to get more informed takes.

Edit: same for IRAN work on the engine.

 

[RyanB]

I don't see it as a problem, I'd want to know what led to that decision. That said, an IRAN is honestly the same thing as an OH, it just doesn't reset the clock, so I'd just want it priced accordingly.

 

[Anthem]

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[Bob Noel]

fwiw - top overhaul doesn't mean anything wrt to extending hours SMOH

I know others will disagree. But I would treat an engine with 1600 SMOH with all new cylinders (0 hours since new) as an engine with 1600 SMOH

 

[ArrowFlyer86]

fwiw - top overhaul doesn't mean anything wrt to extending hours SMOH

I know others will disagree. But I would treat an engine with 1600 SMOH with all new cylinders (0 hours since new) as an engine with 1600 SMOH

Yeah, there's no guarantee it will help prolong the life of the engine. But I've thought of it the same way, if it's 1500 SMOH and 200 STOH, I kind of would expect to price it as 1500 SMOH and forget the TOH even occurred for pricing reasons.

 

[texasclouds]

Seems like he went the cheap route and then sold while he still could. Probably didn’t have forever in mind.

 

[TCABM]

A low time O-320 with a recent top end is different than an TSIO-520 at 1200 hours, which means it depends is the answer and logbook review is in order.

 

[ArrowFlyer86]

Top overhaul before TBO ? Yeah - not a red flag. Having new cylinders isnt a bad thing - especially since some engines just go through cylinder mid-life. But a Top overhaul at 2000 on a 2000hr engine - well, that might not be a good ROI - then again, with the costs of overhauls these days - maybe a top might buy you half a engine life or more for 1/4 the cost.

I kind of thought of it as you must have been having engine issues to justify the TOH.

Especially when I see something 500-700h SMOH but with a TOH in the last 1-200 hours already -- it makes me wonder about the engine quality (or maybe how much the pilot is abusing the engine) if you needed a TOH when you're only 1/4 of the way to TBO.

 

[ArrowFlyer86]

A low time O-320 with a recent top end is different than an TSIO-520 at 1200 hours, which means it depends is the answer and logbook review is in order.

Actually it's funny you use that example. Specifically have one in mind. TSIO-550-C at 1500h and TOH 70h ago.

 

[Checkout_my_Six]

Fresh Top.....meh, it means very little.

 

[Anthem]

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[schmookeeg]

I feel like the 800 hour top OH is just a standard feature of big continental motors. I'm starting to see more rotocoil swaps and lap-in-place in the logs, and I kinda like that signal -- it means they're paying attention or borescoping at oil change, or some other proactive prophylactic methods. Usually means the owner or shop deserve a kudo.

At or near TBO? Yeah that is a strong signal that the owner is a cheapskate or selling due to lack of adequate funds, and the due diligence proceeds to the elbow-deep stage.

 

[Bell206]

As a buyer, how much faith do you put in these TOH in terms of extending engine longevity?

FWIW: Just to keep things in context, there is no "top overhaul." Its a repair nothing more. The question is why did the owner repair the engine.

Do some engines require cylinder repairs more than others? Yes. But as to extending engine life no, in general.

In my experience, outside those engines where short cylinder life is a known feature, any other engine that blows through cylinders has other issues which usually point to the operation side of things.

 

[ateamer]

I kind of thought of it as you must have been having engine issues to justify the TOH.

Especially when I see something 500-700h SMOH but with a TOH in the last 1-200 hours already -- it makes me wonder about the engine quality (or maybe how much the pilot is abusing the engine) if you needed a TOH when you're only 1/4 of the way to TBO.

I’d also question the owner’s knowledge and management of maintenance. It certainly could have been something like a sticking exhaust valve, an A&P whose response to low compression on that cylinder is to top all four, and an owner who just whips out the checkbook.

 

[mondtster]

There are no absolutes when it comes to engine repairs and trying to determine what kind of confidence to place on them solely based on a sale ad and a few logbook entries is practically impossible. In no way would I treat the following cases the same:

1. Engine logs show a field overhaul by a local mechanic using 100% overhauled parts, including overhauled cylinders 8 years prior. Engine ran ~600 hours and the cylinders started having problems, one by one. The owner got tired of repeated shop visits eating into his dispatch reliability so the decision was made to replace all the cylinders with factory new units.

2. Engine logs show a field overhaul by a well known shop in 2007. Engine was installed on an aircraft, run to ~1950 hours and removed for an upgrade in 2016. The engine sat unpreserved until 2022 when it was purchased with the intent to overhaul and install on a different aircraft. The new owner instead elected to replace all the cylinders and install the engine as-is.

3. Engine logs show a field overhaul by a small, reputable, but largely unknown shop. The engine was run 1300 hours over the course of 32 years. Cylinder compressions started drooping, necessitating some intervention. The owner elected to install four new cylinders and re-ring the other two.

4. Engine logs show a field overhaul by a well known shop in 1980. The engine was run 700 hours between 1980 and 2022. New owner started flying the aircraft frequently, and one cylinder's compression dropped to zero. Engine was opened and a failed cam and lifters were found. Cam and lifters were replaced, bearings replaced, two overhauled cylinders were installed, and two cylinders were honed and re-ringed.

Which engine would you like/trust the most? Which one would you like/trust the least? Do you think you'd be able to tell which one is which from a simple log entry and what I've written here?

Another thing to consider is what the engine in question is. If we're talking about a garden variety Lycoming or Continental you can afford to have different expectations than if you're looking at something more obscure and less supported like a Franklin or a Ranger. But, even some Lycomings and Continentals are minimally supported these days.

Personally, I'd be more concerned about who is doing the work and how fastidious they are than I would be with rejecting a prospective aircraft just because it had all the cylinders replaced some time prior to overhaul. But, that said, I expect that you're seeing a lot of "top overhaul" ads showing up because they don't want to divulge how long ago the last overhaul was really done. A recent top overhaul listed in the ad looks better.

BTW, one of the 4 engines listed is on my personal aircraft. I did the work on it and one of the other engines I listed. I personally know the people and the story behind all of them.

 

[chemgeek]

A TOH is kinda weird in my mind. I usually replaced cylinders on condition...one at a time. Replacing all cylinders seems potentially wasteful, or something bad happened to all the cylinders at the same time before it should have.

 

[Clip4]

Depends of the model of the engine and time on the bottom.

 

[camorton]

A TOH is kinda weird in my mind. I usually replaced cylinders on condition...one at a time. Replacing all cylinders seems potentially wasteful, or something bad happened to all the cylinders at the same time before it should have.

I'm with you on this one. Even if 4 out of 6 cylinders need work, why are you taking off the two that are actually working well? Just cause you want them all to look the same? I don't get it.

Really depends on what is wrong, too. Valves, rings, etc.

C.

 

[Kesk]

I'm with you on this one. Even if 4 out of 6 cylinders need work, why are you taking off the two that are actually working well? Just cause you want them all to look the same? I don't get it.

Really depends on what is wrong, too. Valves, rings, etc.

C.

Yes, a top overhaul is pointless. Cylinders should be replaced as needed.

$1500 for an overhauled cylinder with shipping + ~6 hours of labor, more if you factor in a break-in flight. Throwing money after bad.

 

[Checkout_my_Six]

Why replace them all?....well cause sometimes it's just "easier" to do them all. In the case of the Bonanza to get the problem cylinders off (5 & 6)you have to get a few of the others off first....so we now have 4 out of six off....do we do the last remaining two while we're at it? In many cases the answer is ....yes.

And....Gibson's will overhaul a cylinder for $700-800 each. So, that ain't too bad....while yer in there.

And then....to get good cylinder torques on all the hardware it's difficult with the engine installed in the airframe....so, if you remove the engine what else would you do....while it's out? If you're doing this work yourself....it's endless.

 

[FORANE]

Why replace them all?....well cause sometimes it's just "easier" to do them all. In the case of the Bonanza to get the problem cylinders off (5 & 6)you have to get a few of the others off first....so we now have 4 out of six off....do we do the last remaining two while we're at it? In many cases the answer is ....yes.

And....Gibson's will overhaul a cylinder for $700-800 each. So, that ain't too bad....while yer in there.

A couple years ago Gibson's was $500 base price per cylinder for the Lycoming o-340's that I sent them. But, when all was said and done I was out the door at near twice that. Still, I believe they did good work and would use them again.

I did all 4 and would do it again. Sure I could have gotten away with just doing the single worst cylinder but there was wear on the others. Given the failure on the one, the peace of mind for doing all of them was worth it to me. After all, it's my butt in this thing flying over mountains at night and sometimes in IMC.

 

[Magman]

A hidden gremlin with cylinders is the bushing in the small end of the

connecting rod. After changing jugs 1 at a time it was time to dig deeper.

If the Bushing checked “ in limits “ and was not replaced at the last

overhaul it is now possible to be “ out of limits”.

This allows the piston to cock slightly and wreaks havoc on cylinder walls.

 

[Checkout_my_Six]

new pins can help with this...

 

[Kesk]

Why replace them all?....well cause sometimes it's just "easier" to do them all. In the case of the Bonanza to get the problem cylinders off (5 & 6)you have to get a few of the others off first....so we now have 4 out of six off....do we do the last remaining two while we're at it? In many cases the answer is ....yes.

And....Gibson's will overhaul a cylinder for $700-800 each. So, that ain't too bad....while yer in there.

And then....to get good cylinder torques on all the hardware it's difficult with the engine installed in the airframe....so, if you remove the engine what else would you do....while it's out? If you're doing this work yourself....it's endless.

looks great, what paint did you use?

 

[Checkout_my_Six]

looks great, what paint did you use?

The primer was a chromate and the color was a two part epoxy from the local auto paint shop.

 

[bluerooster]

Year before last, went in for annual. Compression revealed 76, 78, 76, 74. SMOH ~500, pulled two jugs and sent them out for overhaul.
One because exhaust flange was eaten away due to exhaust leak, and beyond our ability to repair in situ. The other, I don't remember the reason, mabe it was the 74, and we had to get "creative" to achieve that.

 

[OneCharlieTango]

It’s possible — likely, even — that a TOH is no more interesting than a mag overhaul. Cylinders are basically accessories and they sometimes need replacing.

 

[Steamflyer]

Data point: 2x TIO-520. Top at 1400 hours due to Superior AD. No cylinders removed up to that point. New Superior sets installed and aircraft sold at 2300 hours. All compressions over 70 and oil burn 1in 20 at that point with 900 STOH.

Cranks are both expensive and in short supply, potentially increasing listings of recent tops vs. full overhauls.

That said, wold prefer to top engine as owner, rather than pick up someone else's 10 or 20 SMOH as then can control the break in process. And as always, engines corrode out faster than they wear out.

 

[RyanShort1]

I'm with you on this one. Even if 4 out of 6 cylinders need work, why are you taking off the two that are actually working well? Just cause you want them all to look the same? I don't get it.

Really depends on what is wrong, too. Valves, rings, etc.

C.

There IS a reason for this. If you are down time for 4 of 6 cylinders and can afford to replace the other two, the theory is that you are risking *less* future down time for the other 2.

 

[mandm]

When I did the prebuy on the Arrow, there was 2 leaking cylinders, probably around 1400SMOH IIRC. The last major overhaul was done a LONG time ago, but the airplane has treated me well, I’ve put on hundreds of hours, and I hope to get hundreds of more hours too on this engine. Will monitor it and go from there.

 

[camorton]

There IS a reason for this. If you are down time for 4 of 6 cylinders and can afford to replace the other two, the theory is that you are risking *less* future down time for the other 2.

I disagree with this thinking completely. In my experience, if cylinder one fails prematurely it will have zero impact on the odds of any other cylinder doing the same. There are so many variables in the operating conditions between the individual cylinders on a given engine including fuel and air distribution, baffling/cooling, position on the case, etc. Also each cylinder may have a different history and time in service - set up by a different tech, installed by a different mechanic, with parts made by different manufacturers. Even the largest shops still build these engines by hand, so there will be minor variations just due to the human element.

Even more significant is the possibility of maintenance induced failures. "If it ain't broke, don't fix it" are truly words to live by in GA. To their credit, manufacturers constantly strive to improve materials and processes, but unfortunately this all-to-frequently leads to unintended mistakes and defects that result in expensive ADs and SBs for owners. (Ask anyone who has bought an ECi cylinder in the past 10 years.) Another common fallacy is that a new part is inherently better/safer than one that has been a long time in service. Personally, I'd rather fly behind and engine that is 500 hours past TBO (properly maintained and inspected) than one that is 10 hours SMOH.

You may have heard about some folks who got on a 737 Max 9 up in Portland last week. They probably felt super-safe since the plane was only a couple of months old...

C.

 

[RyanShort1]

I disagree with this thinking completely. In my experience, if cylinder one fails prematurely it will have zero impact on the odds of any other cylinder doing the same.

You can disagree with the thinking I’m just explaining why some owners make that choice. My crew flies about 500 hours a year and we’ve gone through three engine changes in two aircraft that I manage and my boss has swapped engines at least twice that I can remember so our sample size is small but bigger than the average owner. You are correct that is particular cylinder may have a problem at any point but if you know that one cylinder is say 30% away from the end of its expected service life, then if you are pulling the engine to replace three or four other cylinders - even if it only goes to 50% of its service life, the newer cylinder should keep you from extended down time for maintenance quite as fast, and maybe you can even keep the cylinder that was still running OK on the side as a spare.

 

[Steamflyer]

Consider cylinders bolt-on disposable units. A single bad cylinder, sure get a new one for that location. Two bad on a side - three new ones go on that side. 4/6 bad, top overhaul with new cylinders. Once you've pulled 4 cylinders, the maintenance induced risk is baked in IMHO. Then break them in by a pilot who knows how and will take the time to do it right.

Dispatch reliability and safe performance are my boundary conditions. If you fly 40-60 hours a year, maybe the calculus is different, because you are more likely to have bad cylinders in the first place. At 200+, they seem to perform quite well. Recall warranties expect 40 hours/month. So there is that benchmark.

I once read that Cape Air ran their turbos (with FSDO approval of course) to 2800 hours and typically changed a single cylinder per engine along that route. At 500+ hours per year of course.

 

[camorton]

I still don't see how fixing things that aren't broken is going to improve dispatch reliability or minimize downtime. Seems like that would do the exact opposite. If you're managing a fleet, you're probably trying to run a business and hopefully eek out a profit. I don't see how wasting money helps with that either.

C.

 

[Kesk]

I still don't see how fixing things that aren't broken is going to improve dispatch reliability or minimize downtime. Seems like that would do the exact opposite. If you're managing a fleet, you're probably trying to run a business and hopefully eek out a profit. I don't see how wasting money helps with that either.

C.

It doesn't. There's no purpose at all in changing cylinders that are fine. All the moreso if you are changing OEM cylinders with non-OEM overhauled cylinders as those are the cheapest.

You are usually at minimum throwing away $2000 in labor/parts if you add on a cylinder unnecessarily. That's $2000 that could be going towards your overhaul which actually is going to be worth the cost. In my experience you often don't have to change cylinders until the engine is at or beyond TBO anyway, so companies doing it save the cash by pushing engines past TBO if they don't have to based on what FAR they operate under. Of course not all cylinders go out after TBO, but most I've changed were high time. It's almost always due to exhaust valves.

 

[Bell206]

I still don't see how fixing things that aren't broken is going to improve dispatch reliability or minimize downtime. Seems like that would do the exact opposite.

It boils down to the downtime costs vs the part costs. So it specifically depends on the type operation you run. For your Part 91 private recreational ops I usually didn't recommend changing all cylinders on a side unless you had trending data for those particular cylinders. For a Part 91 flight with rental income it would be different but specific to that ops. For example, some engines require the removal of the same items regardless you replace 1, 2, or 3 cylinders. So if all those cylinders were in their trending window of replacement then your money ahead from a downtime cost to change all of them on that side.

 

[Steamflyer]

I've done four top overhauls in the last 10 years (22 total cylinders, all Superior on NA and turbo'd Continental engines) and never had a cylinder issue thereafter. So for the use case of reliability found that just doing them all at once worked. Times after overhaul range or ranged from 500 to 1200 hours (the turbo ones). Agree that in a rental plane it may make more sense to fix one or two and get it back making money. That I never figured out how to do with an airplane!

 

[RyanShort1]

In my case, we fly roughly 10-12 hours minimum a week on aerial photography. If we have to replace two cylinders, we probably have a bigger issue since we run them very carefully and have JPI data, etc… usually it’s pretty rare and one at a time. But, if I thought that the lower end was ok but for some reason we needed to replace three cylinders, I’d definitely be considering replacing the fourth. You already have the costs of downtime and the risks of the break in to consider, and whatever caused more than 50% of the cylinders to be ready for replacement was probably hard on the other cylinders, too.

So, if a factory reman had 2-3 cylinders go bad at say 11-1200 hours, I’d be concerned that the likelihood of that other cylinder having a QC issue before hitting 2000 and thus the sunk cost of an extra cylinder (that in reality is not totally lost as a spare on the shelf) is probably a better value than the potential for a week or more of downtime plus lease or rental costs to maintain contracts.

I don’t think that our margins are huge, but we are a for-profit company doing aerial photography for I think close to 20 years - over a decade of which I’ve run a crew. I’ve also been around flight schools for the better part of 20 years so that’s where I base my opinion.

 

[3393RP]

In my case, we fly roughly 10-12 hours minimum a week on aerial photography. If we have to replace two cylinders, we probably have a bigger issue since we run them very carefully and have JPI data, etc… usually it’s pretty rare and one at a time. But, if I thought that the lower end was ok but for some reason we needed to replace three cylinders, I’d definitely be considering replacing the fourth. You already have the costs i ok f downtime and the risks of the break in to consider, and whatever caused more than 50% of the cylinders to be ready for replacement was probably hard on the other cylinders, too.

So, if a factory reman had 2-3 cylinders go bad at say 11-1200 hours, I’d be concerned that the likelihood of that other cylinder having a QC issue before hitting 2000 and thus the sunk cost of an extra cylinder (that in reality is not totally lost as a spare on the shelf) is probably a better value than the potential for a week or more of downtime plus lease or rental costs to maintain contracts.

I don’t think that our margins are huge, but we are a for-profit company doing aerial photography for I think close to 20 years - over a decade of which I’ve run a crew. I’ve also been around flight schools for the better part of 20 years so that’s where I base my opinion.

It makes sense to me. If the fourth cylinder were to fail in a week or a year doesn't matter. Twelve hours a week generates more money than that forth cylinder replacement cost. If the plane is down to replace known failed cylinders, change the other one, and a week or more of downtime (the mechanic doesn't consider your emergency his as well) plus rental of a replacement can be avoided. You obviously have a database that informs the decision.

Dispatch rates matter for an aviation related business, not so much for the quasi-recreational flyer.

 

[RyanShort1]

Dispatch rates matter for an aviation related business, not so much for the quasi-recreational flyer.

This is the bottom line.

A very high percentage of aircraft aren’t flown anywhere close to 100 hours per year so a week or five down means nothing.