Topped Engines and Recommended TBO

[kontiki]

Looking through ads, I see a lot of airplanes that have been topped before recommended TBO.

I usually assume they were over-leaned maybe ruining a valve.

So once that happens, and the cylinders are made good, is the lower end still good to go to recommended overhaul?

At a recommended engine overhaul on a properly operated engiine, where is the wear worst, the cylinders or the lower end?

Does topping it really justify extending the next overhaul?

Thanks,

 

[bnt83]

Ever thought which parts of the engines corrode the fastest? Start comparing the months since major overhaul and you will learn there are many engines that haven't been apart in 25 years and to get them running without blowing oil all over everything the rusty cylinders gotta go.

 

[bnt83]

Typical piston GA airplane ad: (hypothetical)

TSMOH : 960 hours (2000 TBO)
Topped 100 hrs ago

What they don't say was the major overhaul was in 1975 which is well over the recommended 12 years (usually)

 

[bnt83]

As far as bottom making TBO, it's a crap shoot. It's allways a crap shoot. There are factory remans under 12 years old that crap cams and lifters long before making the hourly TBO.

 

[Fearless Tower]

What kinds of planes/engines are you looking at?

For some engines (Continental 470s, for example), it is not unusual to need a top before making TBO.

 

[poadeleted20]

Looking through ads, I see a lot of airplanes that have been topped before recommended TBO.

I usually assume they were over-leaned maybe ruining a valve.

I know of no reason to assume that. I think loss of compression due to ring/wall problems is probably as common or more common a reason why cylinders are replaced or overhauled before recommended TBO. Corrosion inside a disused engine is another big one.

So once that happens, and the cylinders are made good, is the lower end still good to go to recommended overhaul?

No way to say without examining the engine and knowing what led to the cylinder replacement/overhaul.

Does topping it really justify extending the next overhaul?

Not in and of itself.

 

[NineThreeKilo]

I've seen folks crack up jugs on a poorly managed decent (over 2 degrees a sec kinda thing).

Way I see it, figure best case it'll just make the overhaul cheaper if the top is fresh.

 

[Tom-D]

these cylinders sat at AWO for 20 years unattended, cylinder wall corrosion is a myth, it will cure its self when run.
major portion of cylinders are removed from service for valve problems, not cylinder wall wear.

 

[Tom-D]

I've seen folks crack up jugs on a poorly managed decent (over 2 degrees a sec kinda thing).

Let's see you prove that.

 

[JHW]

I've seen folks crack up jugs on a poorly managed decent (over 2 degrees a sec kinda thing).

BS. Only if they had a liquid nitrogen sprayer cooling them.

 

[ronnieh]

Yep, OWT.

 

[JHW]

Ask your local DZ.

Exactly. Full power to 12k ft. Idle to the ground. Repeat 20 times a day. No preponderance in cylinder problems vs any other engines with crappy continental cylinders.

 

[NineThreeKilo]

Exactly. Full power to 12k ft. Idle to the ground. Repeat 20 times a day. No preponderance in cylinder problems vs any other engines with crappy continental cylinders.

Not exactly

 

[NineThreeKilo]

Exactly. Full power to 12k ft. Idle to the ground. Repeat 20 times a day. No preponderance in cylinder problems vs any other engines with crappy continental cylinders.

Where were you doing 20 loads a day in a piston??

 

[Ted]

Not exactly

On a IO550

Full power prop forward, cowl flaps open, 50rop to 13 or 17.9k.

Decend, power to 15, prop to 2100, cowl flaps closed, mixture to 50rop, keep the EGTs up to keep the CHTs from dropping too quick.

Works great for when we were doing it.

Doing 50 ROP like that? What cylinder salesman told you that was a good idea?

OP: lots of engines end up with a top overhaul. Not necessarily a problem. There's a lot to consider with an engine, way more than I can put in a post (even though others will try...).

 

[bnt83]

these cylinders sat at AWO for 20 years unattended, cylinder wall corrosion is a myth, it will cure its self when run.
major portion of cylinders are removed from service for valve problems, not cylinder wall wear.

Maybe cylinder corrosion is as hit & miss as everything else then. Corrosion pitting shows up the best after letting a freshly honed and washed cylinder sit for a few days and the pitting will turn rusty red first.


Also, the pictures you show, they are pretty dirty so how can you say they corrosion free after sitting 20 years? Also, are they chromed? I can't really tell.

 

[N801BH]

If it a low powered motor like a 150-160Hp 0-320 or a 180HP 0-360 and it had the oil changed often and flown some, then the bottom end will go 4000+ hours. So, if it were mine I would top it and see what happens... if the bottom end does go TU then you have fresh cylinders to put on a rebuilt bottom end... ya really can't go wrong.... IMHO..

 

[wabower]

You can safely bet that most tops were premature if all the jugs were done at one time. The "we might as well do them all" syndrome seems to be pervasive, when re-working only the bad ones is much more practical and cost-effective. Read some of Mike Busch's articles about "cylinders are accessories and should be treated accordingly" for a good overview of this issue.

 

[bnt83]

Sometimes it makes sense to do one bank at a time. If 1 doesn't pass the mustard and the other one (4 cylinder) or two (6 cyliner) on the same side are low, you alreay have the exhaust & baffles off on that side (if two seperate mufflers).

 

[dell30rb]

these cylinders sat at AWO for 20 years unattended, cylinder wall corrosion is a myth, it will cure its self when run.
major portion of cylinders are removed from service for valve problems, not cylinder wall wear.

I dunno, I had to beat a corroded piston out of a cylinder with a block of wood and a sledge once. Old jet ski. It was a tortured old thing though, not a plane sitting in the corner of a hangar.

The bottom end was nearly siezed as well. I bought a gallon of WD 40 and filled up the crankcase, let it sit overnight. Next day I turned it over by hand until the bearings felt smooth. Dumped the WD 40 out and flushed again with WD. Put it back together with a new carb gasket kit, new piston and cylinder. Ran great for three years until I sold it.


That said if a plane had been sitting for 20 years i'd throw some WD40 in the cylinders, change oil, turn it over a bunch by hand and then fire it up and run it. It might check out fine.

 

[ScottM]

Looking through ads, I see a lot of airplanes that have been topped before recommended TBO.

I usually assume they were over-leaned maybe ruining a valve.

You should not assume that.

My engine has had two top overhauls. Never for a valve nor over leaning. Both times it was because of ADs on the ECI cylinders. The first AD required replacement of the cylinders. The 2nd AD on the brand new ECI clyinders required one to be replaced and the others to have 50 hour inspections. I opted to replace all of them with OEM instead of having three suspect jugs on the engine.

So my words to you are when you see engines that are topped, look a little into why that happened. My experience was not unique.

 

[Ted]

You can safely bet that most tops were premature if all the jugs were done at one time. The "we might as well do them all" syndrome seems to be pervasive, when re-working only the bad ones is much more practical and cost-effective. Read some of Mike Busch's articles about "cylinders are accessories and should be treated accordingly" for a good overview of this issue.

One exception would be my friend's RAM T310R. He had all but a couple cylinders cracked on the last annual and opted to do all 12. In that case I think it was logical, given the amount of work that had to go into doing all the others and a safe presumption that those two would need to be done next year.

Cylinders are a real case of "it depends."

 

[poadeleted20]

I've seen folks crack up jugs on a poorly managed decent (over 2 degrees a sec kinda thing).

Metallurgical nonsense. To crack the metal of which our cylinders are made by sudden cooling (it's called "quench cracking"), you need cooling rates of several hundred degrees per second, and you won't get that unless you plunge the engine into a bath of some very cold water (in which case you've got other, much more serious problems). All that you can do by that "poorly managed descent" is cause the cylinders to contract more quickly than the pistons and create excessive wear on the rings/walls. That said, since you'd like your engine to make TBO, try to keep your cooling rates under 60 deg/min to minimize that wear.

Way I see it, figure best case it'll just make the overhaul cheaper if the top is fresh.

Depends on what sort of overhaul you want. Lycoming factory overhauls include new cylinder assemblies no matter how many or how few hours are on the old ones. Other shops are going to send the cylinders out for the same work regardless of their condition. Some will do an "inspect and repair as necessary" on the existing cylinders. Make sure you talk this over with the overhauler.

 

[poadeleted20]

If it a low powered motor like a 150-160Hp 0-320 or a 180HP 0-360 and it had the oil changed often and flown some, then the bottom end will go 4000+ hours.

Will go that long? Maybe, but I'd be more inclined to say "may go 4000+ hours."

 

[N801BH]

Will go that long? Maybe, but I'd be more inclined to say "may go 4000+ hours."

Pull a bunch of them apart as see how overbuilt they are... 1/2 HP per cubic inch is telling... You keep oil in it and they WILL get to 4000+ hours.. IMHO.....

 

[Archammer]

these cylinders sat at AWO for 20 years unattended, cylinder wall corrosion is a myth, it will cure its self when run.
major portion of cylinders are removed from service for valve problems, not cylinder wall wear.

I was hoping someone was going to say this. It's funny how things seem to get overcomplicated in aviation.
That's why I post engine questions on drag racing forums instead, hehe.


Sent from my iPad using Tapatalk HD

 

[taters]

Ever thought which parts of the engines corrode the fastest? Start comparing the months since major overhaul and you will learn there are many engines that haven't been apart in 25 years and to get them running without blowing oil all over everything the rusty cylinders gotta go.

I agree with this a lot, calendar time is just as important as tach time when it comes to engines. I just sold my 150 re-topped at TBO..its was a factory reman out of the box 4 years ago..... I expect the new owner will easily get a lot more time out of it.

When I bought it ..it was mid-time but with 19 years since an overhaul...it lasted me a whopping 200 hours before the thrust bearing disintegrated.

 

[JHW]

A lot of people have trouble with this concept that it might be entirely proper for the top end to have service intervals different from the bottom end. But really, that is typical of reciprocating engines as a whole.

Take the engine in this photo as an example. In broad terms it will:

• run for X years then change injectors & cylinder heads
• run for Y years then change injectors, cylinder heads, cylinder liners, pistons, rod bearings, roll new bearings into crankshaft (crank stays in place, commonly called in-frame overhaul where engine stays in the vehicle)
• run for X years, change injectors & cylinder heads
• run for Y years, remove engine and do out-of-frame overhaul including grinding crank

If you look across engines in boats, tractors, oil wells, whatever, you will see some variant of the above. Airplane engines look very much like the rest of the world, everything except the lawnmowers and honda civics that most people are familiar with and mistakenly try to use as a benchmark for all other engines.

 

[Ted]

Of course, the other question comes down to the incremental value of parts and labor. For something like an aircraft engine where to change main bearings you have to split the case, it's somewhat silly to then not at least look at the crank and consider something be done with the cylinders.

I'm planning on changing rod and main bearings on my Mitsubishi in the next few months. In that car, the bearings don't end up lasting as long as the rings, mainly due to oil starvation issues (which I've fixed).

 

[poadeleted20]

Pull a bunch of them apart as see how overbuilt they are... 1/2 HP per cubic inch is telling... You keep oil in it and they WILL get to 4000+ hours.. IMHO.....

You willing to bet the price of an engine on that with all takers?

 

[Tom-D]

I dunno, I had to beat a corroded piston out of a cylinder with a block of wood and a sledge once. Old jet ski. It was a tortured old thing though, not a plane sitting in the corner of a hangar.

The bottom end was nearly siezed as well. I bought a gallon of WD 40 and filled up the crankcase, let it sit overnight. Next day I turned it over by hand until the bearings felt smooth. Dumped the WD 40 out and flushed again with WD. Put it back together with a new carb gasket kit, new piston and cylinder. Ran great for three years until I sold it.


That said if a plane had been sitting for 20 years i'd throw some WD40 in the cylinders, change oil, turn it over a bunch by hand and then fire it up and run it. It might check out fine.

Did you notice the oil in the cylinder in picture 2?
1 stroke of the piston took out 75% of the rust, and that was by hand with no pressure in the combustion chamber.
I oiled the cylinders before I started disassembly of the engine, and only rotated each cylinder to top dead center to remove them, and then took those pictures.

 

[Tom-D]

You willing to bet the price of an engine on that with all takers?

As you know I deal in the 0-300 / 0-200 mostly, and often see engines that are over TBO. Knowing how well the lower end of these engines are built, I would not hesitate to run either to 4k hours.

The engines I over haul are all over TBO and in nearly all the main and rod bearing clearance are with in service limits.

That said, what I do see is the wear on the lifter bodies, gear teeth, wrist pin bushings, and corrosion in the lower oil sump.
Yes I do recommend overhauling the lower end when you do the top, not because I think it will fail prior to wearing out the next set of cylinders, It's simply a case of knowing everything is right, when your sweet cheeks are over all the cold water up here.

 

[Tom-D]

Metallurgical nonsense. To crack the metal of which our cylinders are made by sudden cooling (it's called "quench cracking"), you need cooling rates of several hundred degrees per second, and you won't get that unless you plunge the engine into a bath of some very cold water (in which case you've got other, much more serious problems). All that you can do by that "poorly managed descent" is cause the cylinders to contract more quickly than the pistons and create excessive wear on the rings/walls. That said, since you'd like your engine to make TBO, try to keep your cooling rates under 60 deg/min to minimize that wear.

That was the common thinking of about 50 years ago.

Manufacturers are now realizing the head to barrel seam is a failure point due to the thermal expansion rates of aluminum to steel, thus the same recommendation to limit the CHTs not the cooling rate.

just saying.

 

[poadeleted20]

That was the common thinking of about 50 years ago.

Manufacturers are now realizing the head to barrel seam is a failure point due to the thermal expansion rates of aluminum to steel, thus the same recommendation to limit the CHTs not the cooling rate.

just saying.

Splitting a dissimilar metal seam I'll buy, but not cracking the steel barrel.

 

[Skylane81E]

Splitting a dissimilar metal seam I'll buy, but not cracking the steel barrel.

That is what the mechanic who worked on jump planes said saw,

Cracks in the aluminum head at the top of the barrel.

Shock cooling was not the limit of the abuse they saw though.

 

[Ted]

So call it what ever you like, but it sure as he11 works for meeting and exceeding TBO in our engines and every piston I've been behind.

50 ROP is pretty much the worst operating point you can be at. 2 deg/sec? 50/min is more like what you want.

If you're operating a 550 as you original stated like that, in the climb, I call BS on cylinder life and going past TBO. On a parallel valve 540 I'd believe it, simply because those things are pretty much bulletproof.

 

[Ted]

But what do I know, I just do this chit 6-7 days a week for a living.

You've made it pretty clear on more than one occasion that, like others who do this 6-7 days a week you don't have nearly as much of a clue as you think. Most people I've seen who fly for a living really have no clue what's good for engines, and you're lumped in the category.

100 ROP is way different than 50, and that's believable.

 

[JHW]

But what do I know, I just do this chit 6-7 days a week for a living.

precious little, as far as I can tell. But what do I know, I only design engines for a living.

 

[NineThreeKilo]

It amazes me the authority someone with little operational experience will speak.

Ok you design engines, great, we are talking aviation engines right??

How many hours are you logging behind aviation engines a month??

 

[NineThreeKilo]

Exactly. Full power to 12k ft. Idle to the ground. Repeat 20 times a day. No preponderance in cylinder problems vs any other engines with crappy continental cylinders.

And you design engines??

So just pull the power and drop from 12k to say a few hundred feet and nothing happens??

You really should send a letter to JPI, every DZ and most all professional pilots of your findings, it'll change the industry