Buying - low use engine?

bflynn

bflynn

Final Approach
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Display name:
Brian Flynn
So I'm still in that realm of trying to figure out how to have a $2000/month slush fund so I can buy AND fly. While working on it, I'm also drooling over a few planes.

How concerned are you guys about engines with high time since overhaul and low hours? At what point is it too low? If it's low, does recency of flying really matter?

Let's say 10 years since overhaul and 1000 hours on the engine - that's 100 hours/year or about 8 hours a month. Seem ok to me, but you never really know.

Does that change if the hours/month goes way down? Let's say 20 years SMOH and 500 hours on the engine for 25 hours/year or 2 hours a month? But logs show it's flown a lot over the past few years, which means it sat for quite a bit before that.

What's your thoughts?
 
I think you're over thinking this......ain't no way of planning it down to that nat's arse.

Me? I'd forget the calendar time since last OH. I "think" that's effectively irrelevant. Anything over 1,000 hrs is good time....and short of replacing a cylinder here and there it should be random maintenance things that will begin to accumulate....then closer to TBO times maintenance will become more frequent to tolerate....then you'd start thinking about an OH to "reset" everything.
 
Its not so much the time, but the frequency. Flying 2 hours a month might not be bad, if it is VERY regular: ie, an hour every two weeks, year round. The opposite could be 100 hours a year, but those 100 hours are flown ONLY in June, July, and August, then the plane sits from September to May (its very common on grass fields in northern climates). I'd prefer the regular 2 hours a month vs 100 hours for 3 months with 9 months idle.
 
Best to buy a high time, it will be priced as a runout, at worst you break even and O/H in first year, or you can win the lottery and it lasts 5+ years.
 
Properly maintained engines can be stored for years with no issues while regularly flown engines can have cam failures due to corrosion. The one truth about every engine is that they start aging and wearing out the day they're assembled. Some age faster than others. Just like people. Consider the engine accordingly.
 
teejayevans said:
Best to buy a high time, it will be priced as a runout, at worst you break even and O/H in first year, or you can win the lottery and it lasts 5+ years.
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That implies you fly it until it stops spinning while you are in the air.... or something horrible enough happens on the ground that forces you to OH.

I'm in this dilemma right now with an engine that's 600+ hours over TBO but running fine and good oil analysis. But I don't really want to run it until it breaks while I'm in the air.....
 
what's to worry with?.....if the oil pressure is good and it isn't making metal....roll on bro. They don't just disintegrate at TBO...
 
Yeah, that's what my mechanic tells me. But he only flies with me one a year (I make him fly first flight after the annual lol)

Oil pressure is pretty low, but it's well in green at cruise power. I don't think it hits green until you give it a significant bit of juice though. I'm not even sure it's in the green during runup at 1700, but I'll pay closer attention next time.
 
Salty said:
Yeah, that's what my mechanic tells me. But he only flies with me one a year (I make him fly first flight after the annual lol)

Oil pressure is pretty low, but it's well in green at cruise power. I don't think it hits green until you give it a significant bit of juice though. I'm not even sure it's in the green during runup at 1700, but I'll pay closer attention next time.
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Have you had a leakdown test done recently? What does that show? If you have good oil pressure and good compression, you should be able to continue on.
 
FormerHangie said:
Have you had a leakdown test done recently? What does that show? If you have good oil pressure and good compression, you should be able to continue on.
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Dunno what a leakdown test is.
 
Salty said:
That implies you fly it until it stops spinning while you are in the air.... or something horrible enough happens on the ground that forces you to OH.

I'm in this dilemma right now with an engine that's 600+ hours over TBO but running fine and good oil analysis. But I don't really want to run it until it breaks while I'm in the air.....
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Don't you have the same concern with a 100 SMOH engine that is running fine and has good oil analysis?
 
Top end has good compression. Have had top overhaul but the bottom end hasn't been open since 1965
 
Salty said:
Dunno what a leakdown test is.
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Compression test.

What's the most common comment after an owner removes a running TBO or TBO+ engine and replaces it with a fresh one? Smoother and more power.
 
rsleeds said:
Don't you have the same concern with a 100 SMOH engine that is running fine and has good oil analysis?
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Not really the same as an engine that's been around for 52 years.
 
teejayevans said:
Best to buy a high time, it will be priced as a runout, at worst you break even and O/H in first year, or you can win the lottery and it lasts 5+ years.
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Then you have to deal with sellers who won't price the engine replacement into the plane. I cannot count the number of airplanes I've seen where the sellers discount an airplane by 5k below the average because the engine is 500 hours past TBO, especially with smaller aircraft. Do they expect someone to buy their airplane at full price and then drop 20k into them for a new engine?

What I'm hearing is that it depends more on the state of the engine than the time. High times and/or long time between overhauls can indicate sitting (and rusting), but ultimately it's the compression test and screening for metal in the oil filter than tells the story.
 
For an engine that has been sitting, you should try to get a borescope and pictures of the cam lobes and lifter faces. This takes pulling a jug; on a Lyc. IIRC it is the left front. From there the borescope can be snaked to the places it is needed. I was just involved in a deal where the elderly engine seemed good, leakdown, OP, etc. but the cam and lifters were rusty from disuse. Seller wouldn't reprice to reflect the junk engine, so the deal blew up.
 
Salty said:
That implies you fly it until it stops spinning while you are in the air.... or something horrible enough happens on the ground that forces you to OH.

I'm in this dilemma right now with an engine that's 600+ hours over TBO but running fine and good oil analysis. But I don't really want to run it until it breaks while I'm in the air.....
Click to expand...

I did not imply that at all. You have a number of indications that it's time to overhaul:
Metal in the filter
Metal in the drained oil
Abnormal oil analysis results
Valve opening not to spec because cam is worn down

Oil usage is high, probably a cylinder problem but I think once past TBO if I have to replace cylinder(s), I think I would overhaul? Thoughts?
 
teejayevans said:
I did not imply that at all. You have a number of indications that it's time to overhaul:
Metal in the filter
Metal in the drained oil
Abnormal oil analysis results
Valve opening not to spec because cam is worn down

Oil usage is high, probably a cylinder problem but I think once past TBO if I have to replace cylinder(s), I think I would overhaul? Thoughts?
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Agreed.....once the top end shows signs of weakness after TBO it doesn't owe you much...might as well go for the OH.

But, before TBO mid time, I'd top it and go. In fact I'm doing that very thing on my Bo....as we speak. :D
 
FormerHangie said:
Have you had a leakdown test done recently? What does that show? If you have good oil pressure and good compression, you should be able to continue on.
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Isn't compression irrelevant for engine OH? I think about it, personally, in three parts, each are independent (I think).

1. Cyclinder. Mike Busch says, and I follow his logic, that these are basically accessories. AND they are independent. you should change them if they need it (cracked, not making power, wobbly valves, etc) and you should only change the ones that need it, no need to "top" unless they are all in sad shape

2. Case, crank, cam, main bearings (bottom end, or things you have to split the case to fix). Watch metal making and oil pressure, if these are good, you should be able to keep flying this until one of those things changes. this would lead to a MOH

3. Accessories (hoses, mags, alternator, starters, hoses, etc) - replace/repair as needed. if you want to schedule based on time in service (say for hoses) or hours (say for mags), cool, but those things wearing out shouldn't trigger a MOH

Am I thinking of this right? I can see where at some point you're looking at several soft cylinders, low-ish oil pressure, hard hoses, etc and you say just do the MOH, but otherwise, if the bottom end is good, it seems like you're ahead to fix the stuff that's actually wearing out.

Example. IO 540 - MOH (not including hoses/etc) is like $30k or so
but if the reason your OH is because three cylinders are not great, but oil pressure is good and you're not making metal, then do 3 cylinders at $2,500 each for $7,500? and fly it another 5 years or until something changes.

I don't own a plane yet, so I don't know what I don't know, but this is how i'm thinking about it.
 
MIFlyer said:
Isn't compression irrelevant for engine OH? I think about it, personally, in three parts, each are independent (I think).

1. Cyclinder. Mike Busch says, and I follow his logic, that these are basically accessories. AND they are independent. you should change them if they need it (cracked, not making power, wobbly valves, etc) and you should only change the ones that need it, no need to "top" unless they are all in sad shape

2. Case, crank, cam, main bearings (bottom end, or things you have to split the case to fix). Watch metal making and oil pressure, if these are good, you should be able to keep flying this until one of those things changes. this would lead to a MOH

3. Accessories (hoses, mags, alternator, starters, hoses, etc) - replace/repair as needed. if you want to schedule based on time in service (say for hoses) or hours (say for mags), cool, but those things wearing out shouldn't trigger a MOH

Am I thinking of this right? I can see where at some point you're looking at several soft cylinders, low-ish oil pressure, hard hoses, etc and you say just do the MOH, but otherwise, if the bottom end is good, it seems like you're ahead to fix the stuff that's actually wearing out.

Example. IO 540 - MOH (not including hoses/etc) is like $30k or so
but if the reason your OH is because three cylinders are not great, but oil pressure is good and you're not making metal, then do 3 cylinders at $2,500 each for $7,500? and fly it another 5 years or until something changes.

I don't own a plane yet, so I don't know what I don't know, but this is how i'm thinking about it.
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If I had an engine with weak compression and meh oil pressure, I'd go ahead and get the overhaul. If you wanted to just do cylinders, I can see why, but I'd rather get the whole thing done at once.
 
My concerns are around the fact that it's been 52 years. I'm probably more concerned about things like oil pumps failing as anything else. Also, isn't it more likely to cost a lot more for the overhaul if you run it until it can't hold oil pressure? That pretty much means everything is out of tolerance and will need to be replaced. (I have a response to my own question, but I'm curious what others will say)
 
shelf time doesn't cause metal to fatigue and fail....I'd rather have a 52 year old engine than a brand new one.
 
Checkout_my_Six said:
shelf time doesn't cause metal to fatigue and fail....I'd rather have a 52 year old engine than a brand new one.
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Shelf time certainly can lead to corrosion. The longer it's around, the better the possibility of corrosion.
 
FormerHangie said:
If I had an engine with weak compression and meh oil pressure, I'd go ahead and get the overhaul. If you wanted to just do cylinders, I can see why, but I'd rather get the whole thing done at once.
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If you had weak OP, you're ready for OH anyway (unless by "meh" you meant marginally acceptable...on the lower end of the range).
 
Salty said:
My concerns are around the fact that it's been 52 years. I'm probably more concerned about things like oil pumps failing as anything else. Also, isn't it more likely to cost a lot more for the overhaul if you run it until it can't hold oil pressure? That pretty much means everything is out of tolerance and will need to be replaced. (I have a response to my own question, but I'm curious what others will say)
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I think the only time an overhaul will cost more is if the oil pressure is low enough where the engine starts to make metal (eat itself; self destruct). Otherwise, lower and lower oil pressure doesn't necessarily make a difference- new bearings and turning journals and resizing gets done anyway. Automotive experience only tho...
 
Salty said:
My concerns are around the fact that it's been 52 years. I'm probably more concerned about things like oil pumps failing as anything else. Also, isn't it more likely to cost a lot more for the overhaul if you run it until it can't hold oil pressure? That pretty much means everything is out of tolerance and will need to be replaced. (I have a response to my own question, but I'm curious what others will say)
Click to expand...

I think engine oil pumps rarely fail, just because they are bathed in oil. Do you have an engine monitor? Having a engine monitor greatly helps you to diagnose engine problems and see minor problems before becoming major ones.
 
Salty said:
My concerns are around the fact that it's been 52 years. I'm probably more concerned about things like oil pumps failing as anything else. Also, isn't it more likely to cost a lot more for the overhaul if you run it until it can't hold oil pressure? That pretty much means everything is out of tolerance and will need to be replaced. (I have a response to my own question, but I'm curious what others will say)
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If the engine is worrying you, get it overhauled. Sounds like you've gotten you money out of it already, there's no reason to fly around with an engine that scares you.

455 Bravo Uniform said:
If you had weak OP, you're ready for OH anyway (unless by "meh" you meant marginally acceptable...on the lower end of the range).
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That is what I meant. If the compression is getting down in the lower part of the range and the oil pressure is as well, I'd get it all done at once.
 
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