Top Overhaul VS Major Overhaul?

[CBogle]

O.K. From my thread on Oil Fouling, it appears that I may be in need of a top overhaul. A couple of questions:

1. With 400 hours to go before TBO, is a top overhaul worth the effort or would it be more cost effective to go straight to a major overhaul?

2. I understand that major overhauls on C-145's run around $20k. What is the ballpark cost of a top overhaul?

3. If the oil fouling is primarily limited to one cylinder at this time, should I only top that cylinder, or, will the others soon follow?

At this point, I'd really prefer not to part with $20k.

Any advice would be appreciated.

Thanks,

Curt - N4288V

[tunraflyer]

If it were me and the problem was limited to 1 cylinder I would only top that cylinder. With only 400 hrs till tbo you may have others that will need work before that but i would cross that road when i get there. If it were me I would pull the problamatic cylinder and send to an overhaul shop and have it yellow tagged verse buying 1 new cylinder, they may reject it but they could have it safe to fly for a lot less.

[blueldr]

I'm sure you understand that your engine does not require a major overhaul at TBO if you're operating under part 91.

If the engine, at 1000 hours, had excessive oil consumption, low oil pressure at operating power, poor compression, low satic RPM, etc., all clasic indications of a too tired engine, you would overhaul it because of the CONDITION. --- RIGHT?

If the engine has 1800 hours (TBO) and all indications are that the bottom end is in good shape, why fix something that obviously "aint broke"? Consider the CONDITION.

I've seen Continental O-300 series engine bottom ends go over 3000 hours a number of times if they're properly cared for and have regular oil changes to keep them clean.

I really believe that any properly cared for O-300 bottom end will easilly be able to handle two 1200 hr top ends. It's easy to calculate the overhaul savings for the hours over TBO.

[GAHorn]

If it were me and the problem was limited to 1 cylinder I would only top that cylinder. With only 400 hrs till tbo you may have others that will need work before that but i would cross that road when i get there. If it were me I would pull the problamatic cylinder and send to an overhaul shop and have it yellow tagged verse buying 1 new cylinder, they may reject it but they could have it safe to fly for a lot less.

I agree. I once (on an IO-520 engine) had a cracked valve guide that was simply replaced, the cyl was cleaned up and a serviceable piston with new rings was installed and that engine continued towards TBO just fine. http://www.aircraftcylinders.com/index.htm My cost including labor was minimal (approx. $400.) Another option might be a part-life exchange cylinder. And, of course, a new replacement complete cylinder assy is always correct.

[Bruce Fenstermacher]

Curt

All good advice has been given previous even if you replaced all six cylinders in my estimation.

You can have a plan say to remove one or two of the cylinders for intended overhaul or replacement. Once they are removed the inside of the engine bottom end can be better evaluated for the condition of the rest of the engine.

You can see the cam, pull a connecting rod cap and look at the bearing and so forth.

All before sending for overhaul or purchasing a cylinder.

One thing you must be prepared for though is finding something you don't want to find. Ignorance is bliss but not necessarily safer. It's a tough bullet to take but I always try to remember I'd rather take it on the ground than at 1000 ft.

Another thing in my mind with out any fact to back it up is that catastrophic failure is hardly ever because of the bottom end. More than likely catastrophic failure is set in place by valve train failure or cylinder failure.

[GAHorn]

Just discussing...not arguin'...
I'm not sure I'd encourage exploration in the lower end just because the cylinder may be off.
While it's certainly interesting to look down in there... there's really not much criteria to judge what is being looked at in that condition... There's no measurements that can be taken. There's no way to judge conn-rod twist, or gear lash, or cam wear, or... just about anything at all.
Arbitrarily removing a connecting rod-cap to look at a bearing doesn't seem like a good idea to me. First, you'd likely have to also/unnecessarily remove the opposite cylinder to reach the rod bolts. Then you'd be looking at a bearing without any guidelines or specifications for a part-life bearing. (Unless you go thru all the plastigage excersize, and there's very little to be gained by that.)
And you'd be risking the bearing by possible contamination and/or reassembly errors, not to mention the extra cycle of a re-torque and re-use of rod bolts, locking fasteners...
In short, I don't think there's much to be gained by additional partial disassembly of the lower end and I wouldn't encourage that. If it ain't broke...

[Dave Clark]

Just discussing...not arguin'...
I'm not sure I'd encourage exploration in the lower end just because the cylinder may be off.
While it's certainly interesting to look down in there... there's really not much criteria to judge what is being looked at in that condition... There's no measurements that can be taken. There's no way to judge conn-rod twist, or gear lash, or cam wear, or... just about anything at all.
Arbitrarily removing a connecting rod-cap to look at a bearing doesn't seem like a good idea to me. First, you'd likely have to also/unnecessarily remove the opposite cylinder to reach the rod bolts. Then you'd be looking at a bearing without any guidelines or specifications for a part-life bearing. (Unless you go thru all the plastigage excersize, and there's very little to be gained by that.)
And you'd be risking the bearing by possible contamination and/or reassembly errors, not to mention the extra cycle of a re-torque and re-use of rod bolts, locking fasteners...
In short, I don't think there's much to be gained by additional partial disassembly of the lower end and I wouldn't encourage that. If it ain't broke...

I agree here except it is possible to find problems developing with cam & followers by a visual inspection. I'm sure George meant you just can't measure any wear there. In the O-470 you can replace the lifters without splitting the case and it's recommended to do that at a top if any wear is noticed and if the cam still looks undamaged. I've found cam lobes worn enough to be seen without measuring during a top overhaul and then of course one finds himself into it deeper than anticipated.

Not sure it was mentioned yet but a good indicator of bearing wear is the idle oil pressure when warm. You need to have an idea of what it should be, say from an earlier data point, for it to be real meaningful unless it's just really low. I don't remember if there is a minimum specification on it either.

[Bruce Fenstermacher]

George I realize you aren't arguing. You have very valid points about torque cycles on rod bolts. I didn't mean to imply that a comprehensive analysis of the engine could be made peering through a cylinder hole. But some useful info can be obtained.

I've not seen a bad cam my self but have heard of cases with others that upon removal of a cylinder for cylinder problems the cam was determined to be worn or wearing abnormally from a visual inspection.

I myself against the advice of several competent knowledgeable people removed my engine at about 700 SMOH to track down the source of what was probably negligible metal shavings in the oil screen. I just didn't feel good about it.

I fully expected to find a problem with a piston pin plug or a gear or related equipment in the accessory case. When nothing materialized we nearly reassembled the engine.

It was then suggested that we remove a rod bearing cap just to visually inspect the bearing. I knew it wouldn't be the source of the metal I was looking for and didn't see much point. It would be at best a random look and what was happening with one bearing wouldn't necessarily have any thing to do with another.

At the last moment I relented to the advice and we removed a cap and bearing to find the bearing worn nearly through the babbitt. We were all shocked. We found several others as well as the mains upon the overhaul that proceeded.

There was no indication from oil pressure or oil screen particles that this would be the case. I don't do oil analysis.

Was I lucky, yes. Is this typical, I hope not. Would I pull a bearing cap on a mid time engine if a cylinder was removed specially not personally knowing the history of those hours other than log entries? More than likely, YES.

[HA]

if you have a cylinder off, I would always look into the depths to see what mayhem might be visible - and if you see a worn cam lobe or two (I mean SPALLED with chips of metal missing, or a lobe worn away until it's round, etc) then yay - you averted disaster and can spend the kids' inheritances on an overhaul.

I wouldn't pull a rod off while I was looking around unless it made clunking noises from all the excess clearance when I yanked on it, but Bruce has a unique experience that colors his thoughts there and that's fine too. I did once pull a jug off of an engine to find that the "overhauler" had had used the wrong rod nuts (old-style Lyc castellated nuts) and not put the required safety roll pins in, so you can sometimes find unpleasant surprises with just a good visual check. why not look everything over a bit while you're there.

[4stripes]

Two years ago my oil pan rotted through. It was noticed that one cam lobe was starting to wear so the bottom end was done, with new cam. One year later, I ended up with a couple cylinder problems, so I bought 6 new Milleniums (new top). Costly, but I wanted reliability, not a hangar queen.
In hindsight, which is always 20/20, I should have done a complete O/H. My engine was at 1000 hours, but was O/H about 30 years previously.
Food for thought.
Cheers Eric

[Dave Clark]

Well I'm into a top on the O-470J now and I found one spalled lifter. My engine shop old-timer-guru-friend said if the lobes look good AND if your fingernail doesn't hang on the edge of the cam lobe (metal rolling off the edge) the cam is ok (not worn through the hardening) and just put in new lifters/followers so that's what I'm doing. The lobes look great. Too bad the C-145 needs to have the case split to replace the lifters.

[MoonlightVFR]

c Bogle

I am curious as to how many hours on your suspect cylinder?

Do you have standard size cylinders or was engine bored out .010, years ago?

I am all for building a knowledge base on FORUM then throwing that at problems instead of your DOLLARS!

[CBogle]

Grady:

Without further research, I don't know for sure what was done to the cylinder's at top. The engine is about 1200 hours since major overhaul, and, 600 hours since top overhaul. At the top, the cylinders were chromed. I'm guessing that if they were chromed, they might have been bored.

Just a footnote...I think I was pre-mature (hopefully) in thinking that I might need a top overhaul, or even a fix on just this one cylinder. Since I've been leaning during taxi, I've had no fouling problems. Since leaning on the ground has stopped my fouling, I believe that I can also eliminate leakage from the primer as a cause of the fouling as I assume that leaning would have no effect on primer leakage. In other words, even leaned, if leaking, the primer would still leak into the cylinder and foul it. (However, I could be wrong here too if a primer leak was adding just enough fuel to cause fouling when rich, but not enough to foul when lean. In other words, I can't be sure it isn't a primer leak without disconnecting the primer...which at this point, since eveything seems to be O.K., I don't want to do.)

The only change, and I mean the ONLY change that I am sure of at this time is the switch from Champion to Unison plugs. There was one other member in one of these threads that said he also had fouling problems only after switching to Unison plugs from Champion. Just to completely eliminate the Unison plugs as a factor, I'm going to switch back to Champion and go back to my old practice of not leaning during taxi, and see if the fouling persists or is eliminated. If eliminated, there will be no other conclusion to reach other than the Unison plugs are more prone to fouling in my engine than Champion plugs. If the Champion's still foul, I'll know that something else had to change.

Thanks,

Regards,

Curt

[davevramp]

How do you lean when you taxi, what procedure do you go through?
Dave

[cfiatzph]

You probably did'nt give enough info. How long since the last overhaul? How is everything else? Carb/mags etc? Personally if its one or two cylinders I would just replace and go, when you start going past 3 probably a good idea to overhaul. I had a 0-300A that had good compression ran ok but it had been 35 years SMOH, and the engine leaked like CRAZY. It only had 1000 or so hours on it. You can get a overhaul for around 10k-12k. 20k is a little crazy. I would advise against going much over TBO as I did this once in a 0-200 and had a piston come apart in 4 pieces, just not a good idea.