Looking for advice-used Lyc
 

This is a question for some of you folks who know engines well. How would you feel about 2000 hours on the first factory reman being removed from a Piper trainer? It's flown the hours over the past six years and a mechanic I trust will borascope, compression check and oil sample for me. I can get it for Lycoming core value. Are there other things that should be checked? Should it be topped, just because? If it passes the tests, how long could I reasonably expect to run it before overhaul? Obviously, I'd prefer a brand new engine but money prevails.

Thanks, JL

2000 hrs
 

The manufacturer gives 2000 hrs as time for "overhaul", and for good reason. It is naive to thing that just because an engine runs, and has "good compression" that it is a safe and healthy engine.
Given the nature of these dinosaurs, the manufacturer'sd recommendation should be headed.
As for the boroscope inspection, it would give you some indication of condition, but it would not give you the amount of wear on the cylinder bores. Differential compression, especially after warming up an engine is also a very poor indication of engine life expectency. A cold test (after sitting overnight) would give a better indication and it is still not conclusive.
A 2000 hour engine will indeed have wear, but differential compression is performed at TDC, where there is little if any wear. Add to that oil after a warm up, and your will be lied to.
The automotive method is a far better indication. Measuring given pressure after the engine is spun in pre determinded amount of time or "spins". In this method, pressure has to be built up to a predetermined number for a predetermined number of engine revolutions. Example" 150# after 2 to 3 rpm. If the build up does not take place in that time, engine wear is suspect.
Spinning it longer will (or may) eventually build up to specification, If so, it will tell one that the bores are worn, or rings are worn or pistons in poor condition. Never come up to specs, valves.
In the auto world, after a lenghty spin, oil may be squirted into the cylinders. Should pressure rise quickly, wear is obvious.
Bottom line, in aviation differential compression after a warm up proves absolutely nothing as far as engine health is concerned.
As to oil sampling, unless is has been recorded from day one, they to are useless.
These engines operate with tolerances where auto engines, lawnmower, motorcycles, and even locomotives, would be ready for the scrap yard.
Gears used are straight cut and with larger than required pitch. They need substantial backlash, and oil anylisis, will verify with the ever presence of steel. Pistons with .010 to .012 when new to a maximun of .020, will be spewing aluminum until the cows come home, hence aluminum will be present.
In any case, the list is long, but boroscoping, compression test, and oil smapling will tell little if any thing about the future of a high time engine.
If the price is right, the fact that it runs is an indication of a good core. It may be a good investment at that.
BTW, don't be confused about the terms overhauled, rebuilt or remanufactured. That's another topic of discussion.
If the price is right, a good rebuild(assembled to new tolerances) may save you lots of bucks.
T88
A&P/IA/EAA Tech Advisor
RV10 with Lycosaurus

Actually,

In the racing world, the leakdown test, identical to the airplane test is considered much more precise and is used by most teams instead of the spinning compression test.

Your argument that differential or leakdown tests prove "absolutely nothing" is simply wrong. To be sure they also do not prove "everything" but they are a reliable test that indicates the health of the engine.

Second, in the other test an engine is spun to see which maximum number it produces, there is no number limit on revolutions. The engine reaches its maximum right away anyway because each revolution all pressure in the bore is released on the exhaust stroke. Those guages actually store the pressure and each time the compression stroke occures it pressurises the guage chamber, until the pressure generated in the stroke is equal to the pressure in the guage chamber...it hence climbs no further.

But no four stroke "Builds up pressure" from one revolution to the next, as it is at zero pressure after the exhaust stroke, and actually at vaccum during the intake stroke.

Many, including the head of the twin cessna club, also an A&P IA will tell you that an engine is most reliable at the end of TBO, and short of evidence of wear, these engines can reliably run well past TBO.

The idea that a single oil analysis is completely useless is similarly wrong. Again it is only one tool, but it is a tools used in all forms of motorsport, and many other areas of transportation.

I see that you are an A&P and tech counseller. Perhaps you just used hyperbole to make your point that even with tests there is risk to using a run out engine. Certainly there is risk.

But your exagerated statements lead me to believe that you are simply repeating what you have heard, but never done in the "car world". In any case your statements are clearly beyond your personal knowledge.

13 years roadracing
FA, FC, FF, F125
Countless air and water cooled racing engines.

Unless you're doing commercial flights, I never really understood the significance of TBO. If the engine runs, and is in decent shape (and especially if you can get it at a great price) I'd just have it closely inspected and run it, knowing that you probably won't make it another 1000 hours.

This may sound very "Zen-like" and flaky but maybe someone will back me up on this... When you take a system (any system...doesn't have to be an engine) that's been happily running for thousands of hours and you take it apart, clean things up, replace things, etc etc etc etc, it's almost like you're screwing with it's karma, or something. I see this time and time again. It's almost like the parts have to get to know each other again, and the old parts are annoyed with the new, shiny parts, etc, sometimes there are teething pains involved and in general nothing's quite right for a while.

Mumbo jumbo? Maybe, but my feeling is that if it's running strong, you give it a GOOD inspection and everything is thumbs up, I'd personally just keep a close eye on it and leave it be until it gave me a reason to tear it's guts apart and rebuild it.

Once again, this isn't any sort of recommendation. It's just what I'd personally do given my NON-AVIATION engineering backround. There maybe some big detail here that I'm missing.

edit: a buddy of mine saw this post and sent me this link. Maybe there really is something to this....
http://www.airbum.com/grassroots/GrassrootsKarma.html

Opinions will vary (obviously already have in the thread), but I would feel better about this engine, with a clean service history, than a 20 year old, 1000 hour engine that had long periods of inactivity. If the engine was running well, and removed simply for regulatory/part 135 reasons, you can make a very good argument for hanging this well proven engine on your new airplane, and not having to worry about new engine break-in issues while simultaneously worrying about the new airframe.

A friend of mine runs a helicopter service, and hangs a new HIO-360 on one of his Enstroms about every 18 months because they have reached TBO (lots of pipeline patrol). I almost bought one of his cores to hang on my RV-8, but found a deal on a 400 TTSN IO-360 instead. I was planning to fly 100-200 hours or so and then take the airplane down for paint and overhaul.

I have run 3 different 6-cylinder TCM engines at least 400 hours past TBO without problems. All were operated consrvatively by a small group of pilots, and flew regularly. If repeated, thorough inspection continue to look good, I wouldnt' worry about it.

I wouldn't "top" the engine without cause. This is (again) just an opinion, but with the cost of new cylinders so close to overhaul, I'd just replace rather than repair the jugs.

James Freeman

Sounds like a deal to me.
 

For my -7A I found a run-out O-360 (in the VAF Classifieds!) for about what a core would be worth. It was originally in a (IIRC) '79 Archer for 2100 hours, then, with compressions still acceptable went into an RV-4 and ran for another 350 hours at which time the owner invested in a new Superior IO-360. The engine was still running fine when pulled. My airplane will be new when completed so I elected to have the engine overhauled to keep everything essentially fresh. With all required parts, labor, and proper inspection of all components I wound up spending about $13,700 including the original purchase price. And the engine is still certified. I can't say you'll be as lucky as I was but I'd certainly consider the engine you're looking at. Hope this helps.

2000hr engine
 

Mr. Jconard.
The leak down testing method of compression testing, is not the most precise method of determining compression. No engine is operated solely at TDC. With the experience you claim to have, I'm certain that you can understand the life of a reciprocating piston in a slopily fitting bore. Hence the accepted sure method of determining in compression pressure is being built up and retained is indeed the spin method. **** it's been correct for more than 90 years.
As to proving a healthy engine with good compression numbers, simply remove the rocker arms from your Lyc, or any other OHV engine, Bring it up to TDC and do a differential test, and surprise, surprise, you will indeed have good compression, but the engine is minus one cylinder, Gawwlly!!
No four stroke builds up pressure? Sir, I suggest you do some homework. A weak cylinder will indeed build up pressure if spun long enough, as opposed to a fresh cylinder will build up pressure immediately. Better read on.
As for those "expert" you mentioned that state that an engine is most reliable at the end of TBO, I guess they must know something more than even Lycoming or Continental must know (not that either are paragons on internal combustion engineering).
Obviously, you have little or no experience with oil anylisis, one ain't gonna cut it. Oil analysis are intended to be judged based on trends, short of a disaster. Those trends begin at birth, not at 2000Hrs.
We do agree that there is risk. I do not use hyperboles to make my point, but simply use common sense that any aircraft engine especially at TBO should not be trusted due to time alone, and for the "methodolygies" used to determining its health.
As to my exagerrated statements, with only 13 years of road racing you experienced, I doubt you can match mine or even come close for that matter.
One more small item, you do not know me, or anything about me, my training, background or experience, so I therefore respectfully ask you to keep your snide remarks to yourself. If you wish to express them, please do so off line.
T88

I'm no Lycoming guru, but have built and run my fair share of auto and bike motors. The leak down test seems like a pretty crappy method for determining bore/ring health, but pretty good for determining valve health. In any case, cylinder wear is pretty far down on my list of concerns for a 2000 hour engine that's keeping me way up in the sky. I'm much more concerned about my rod and main bearings, oil pump, and other junk that'll stop the whole works pronto.

All this talk about saving money on the motor causes me to remind others that banks will finance such things and that the cost of a new zero-time engine is basically money in the bank. Yes, you'll have to write a check every month, but if you ever get real sick of that you could yank the motor, sell it, and replace it with the swap meet version anyway.

JMHO, of course.

What I would do is plan on topping the engine with NEW cylinders. But before you order them, have your mechanic take off the cylinders and inspect the guts of the engine for signs of wear. This will give you a much clearer idea of the cam and other wear items. If all still looks good, slap the new jugs on and run all you want.

contraire mon ami
 

As far as a 2000 hour trainer engine, reman, it will need overhaul sooner than later. Although flown regularly over 6 year service, it may have been ridden hard put away wet. Plus it is at least second run going on three. If they want more than $3,000 I would pass. Reason is you can buy a "non certified" Lyc clone for $19.5k. A comprehensive overhaul can cost $14k. Add core you see you are getting close to new. You can over haul it yourself and save $4k to $6k, but you are looking at least $8k for a do it yourself overhaul. If the core cost $5k plus $8k to $14k for overhaul, depending on who does it, you may save some to no money. However you have a used engine (third run?) you overhauled, with no warranty and used overhauled parts. Make them an offer and see if your A&P friend can help you do the overhaul. Because you are experimental you are not stuck with a particular model engine. You have the option to build a ECI or Superior engine. Also if you go new you may choose a more desirable configuration, such as constant speed prop. You can plug in the crank and use a fixed prop, but it is there for future upgrade if you desire. It will help resale if you sell it later. In this above case they have some records, so may be there are a few hours left on it. So fly it for a year than overhaul it. G

I guess I am naive. I had my O320's on my Piper PA-23 (Apache) going strong at 2200 hrs when I sold it. It continued to fly with the new owners for another 100 hours before I lost track of the folks who bought it. I have heard 2,500 hr reached more than once. I would agree a dusty engine sitting on an old tire, in the corner of a hanger with an inch of dust, would not be a good candidate to go past TBO. Who knows?

"Given the nature of these dinosaurs, the manufacturer'sd recommendation should be headed."

I agree with heeding manufactures recommendations and 2000 hours is the recommend number. However with good compression, good oil analysis and normal operating parameters: oil pressure, temps, CHT's you can continue to fly as long as you like. Recommended TBO only applies to part 135 commercial operators, right. Most important is the engine is flown frequently. That is key. As far as used engines, if you can't run it than you can't really know, unless you have records and reliable info.


The comments you make about differntial pressure test are interesting but you would think the FAA, Lycoming, the who aircraft industry and operators would would come up with something better. It is one small diagnostic tool. A standard if you will. You are saying it does not really measure true piston and piston ring blow-by, I am surprised. I thought it did. Whether in practice it catches all bad rings I can't say. I am sure you can be fooled, but a diff pressure test, plus oil use, aircraft performance, oil analysis and monitoring engine parameters will give you the whole picture. Granted in a used engine sitting on a pallet you can not do all of these things. I have tested an engine for ferry flight with my thumb before. The absolute value is a bit of trick, but we all want high 70's compression over 80. Low 60's is an indication of something. I had a friend who by his own fault burned a valve by intentionally running way to high CHT's. The engine ran fine. However one jug had ZERO/80. It was small smiley burn on the valve. However when running the valve was sealing so the engine ran fine. The Diff Press test will tell you if something major is bad and where to look, valves or rings by listening for air escaping into the crankcase, exhaust pipe or intake side of the head.


Your comments about oil analysis are also interesting. There are normal levels and trends, values are in the order of parts per millions. The case, heads and piston are aluminum, so some will always get in the oil. We are talking about TRACE amounts. There will always be some in the oil. It is only a small part of the picture. You really worry when you see steel or bearing babbitt material in the mix. Again with out good records on a used engine, which is usually the case, a single oil test on an engine not run recently, I agree, is of little value. However as a trend reading it is great. Chances a used engine does not come with detailed log book, many consective recent oil analysis test and monitored like a prize pony.

As far as all the lawn mower and slop tolerances, with all due respect, I don't agree. :D The tolerances on the bottom end, flatness tolerance of mating surfaces are very high. I have an overhaul manual. I hear this from the car guys about piston balance tolerance, which is just not important. We are talking about 2,700 rpm red line, not 6 or 9 grand or 16 grand on a cycle.

The Lyc is an engine engineered by engineers, made intentionally with those tolerances for engineering reasons. Like the crank and piston rod fluid film bearings are similar in all engines. Tolerance is key to producing the oil boundry. To criticize is fine, but understanding it on an engineering level is needed. There are reasons for tight and loose clearances.

This sounds like the same rhetoric that the auto guys talk about. I laugh when my O360 RV-7 passes them at +15 mph burning 1gal/hr less than their RV powered by an engine with superior tolerances. Loose is good sometimes. :p

As far as stright cut gears, what is your point? :confused: All gears have backlash by design. Straight cut as you call them are really "involute" gears, and they are a thing of beauty. There are no real "straight cut" gears since Leonardo Da vinci or Colonial water mills with wood gear days. Involute gear teeth are characterized by high strength, efficency, durability and cost effective. For this application, straight gears as you call them, is the best choice. Even if the Lyc accessory case was designed today. During undergrad work for my mechanical engineering degree, I studied gear analysis/design as one of the subjects in a class called "machine elements".

Have you ever seen a Lyc accessory case gear drive fail? Remeber the gears in the back end of a Lyc have low loads, unlike the ones on a car transmission, gear box or a differntial, with much greater loads. The BIG killer is corrosion pitting from disuse.

No offense, you make a lot of good points, but I don't understand the platitudes about lawn mower tolerance. May be "tractor tolerance".

It is true air cooled engines have greater piston to cylinder wall clearance due to thermal expansion. This does not hurt anything except emissions and blow-by (which does dilute the oil). As far as the "bottom end" the tolerance of the bearings are similar to any engine. The cylinders do use larger tolerances but that is OK. The Lyc has enjoyed many small and large metallurgical and manufacturing improvements over the years. I am surprised to hear you repeat this stuff as if it was a problem.

The Lyc is a mission specific, purpose built engine that was carefully thought out. All "compromises" where chosen to get a final engineered product: A light, powerful, efficient, reliable direct drive aircraft engine. If it was so bad I wounder why two additional companies started manufacturing Lyc clones, and why is the FAA is certifying a version of the Superior Lyc clone? They should have contacted you to find out how to make the engine better. :D

I am kidding, but do you really think the Lyc is needlessly sloppy? Do you think it can't be flown safely and reliably past 2000 hours? I am gussing but you are an alternative engine fan?

George