Engine Compression Question

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Onewinglo
Posts: 114
Joined: Wed Sep 17, 2014 1:04 am

Engine Compression Question

Post by Onewinglo »

Guys,
I’m looking at a 170A with 1072 hours SMOH, and the compressions at last annual were 60, 70, 66, 62, 75 and 70. It has 5804 total time on the engine and airframe.
Do these compressions indicate an engine that’s at the end of its life?
Thanks, JP White
C170A N1714D
My First Airplane!
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pdb
Posts: 466
Joined: Tue Apr 23, 2002 3:39 am

Re: Engine Compression Question

Post by pdb »

Not necessarily. There are a lot of other factors to take into consideration, not the least of which is how often has it been recently flown. If its been sitting in a field but not flown regularly for a while, it could be a problem. Is the engine rusting from the inside?

What you do need to recognize is that there are a couple of cylinders in the low 60's. What this could well mean is that you may be replacing some cylinders in the near future and they won't be particularly cheap. Replacing a few cylinders is not the same thing as overhauling the engine. Will the engine make it to overhaul without replacing a few cylinders before overhaul? Probably not. Could it go another 100 to 200 hours or more before replacing cylinders? Maybe, maybe not, its tough to tell.Its certainly at mid life but with a few new cylinders, its quite possible that it has plenty of hours left before overhaul.

If you factor that into the price of the plane, it may be still worth looking at. What you don't want to do is buy the plane and then be surprised in the next year that you have to buy and replace a couple of cylinders.

What else do you know about the plane? Has it been flown regularly for a while? Has the owner been doing oil analysis? same mechanic for a while?
Pete Brown
Anchorage, Alaska
N4563C 1953 170B
http://farm3.staticflickr.com/2366/2527 ... 4e43_b.jpg
Onewinglo
Posts: 114
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Re: Engine Compression Question

Post by Onewinglo »

Pete,
The owner has passed away. He aparently was unable to fly much for a few years.
Flew only 6 hours between 9/2011 and 3/2013.
Flew less than 4 hours from 3/2013 to present.
It is hangered and gets started and ground run every month or so.
Thanks for the reply!
JP White
C170A N1714D
My First Airplane!
counsellj
Posts: 420
Joined: Sat Apr 30, 2005 2:58 pm

Re: Engine Compression Question

Post by counsellj »

Time for a good borescope inspection and an oil sample. Ground running an engine (to exercise it) is about the worst thing you can do to an engine. It doesn't produce enough heat to eliminate the moisture in the oil. I would expect this engine to be full of rust and possible pitting. What does the inside of the oil pan look like???? I would be very leary of this engine. But if you can get the price down low enough to cover an overhaul, it might be another story. Where in Alaska has this plane been? Good chance the corrosion has had 5 years to grow unabated if it hasn't been getting an good annual each year.

.02

Jughead
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pdb
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Re: Engine Compression Question

Post by pdb »

Time for a good borescope inspection and an oil sample. Ground running an engine (to exercise it) is about the worst thing you can do to an engine. It doesn't produce enough heat to eliminate the moisture in the oil. I would expect this engine to be full of rust and possible pitting.
I agree.

This thing needs to be looked at very carefully. It hasn't been flown enough to have good internal rust protection so there could be some hidden problems. Proceed but proceed with caution and don't get cheap when it comes to having a qualified mechanic inspect the engine. If you're not scared off by the engine inspection, proceed with a satisfactory annual inspection by your mechanic as a condition of sale.
Pete Brown
Anchorage, Alaska
N4563C 1953 170B
http://farm3.staticflickr.com/2366/2527 ... 4e43_b.jpg
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T. C. Downey
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Re: Engine Compression Question

Post by T. C. Downey »

Long storage times do not ruin the engine. With this case I would remove the spark plugs, spray the cylinders with a light oil, crank and spray until the oil coming out the lower spark plug hole shows no rust. replace the plugs, change the oil and filter if it has one. and start normally. run for 5 hours on that oil and change it again. You should be good to go.
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pdb
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Re: Engine Compression Question

Post by pdb »

Long storage times do not ruin the engine.
I agree when the engine has been properly pickled for storage but that's not really the issue here. This plane has only been run about 10 hours in 3 years and probably was not run long or hard enough to drive out the water vapor (combustion product) from the oil when being run on the ground.
Pete Brown
Anchorage, Alaska
N4563C 1953 170B
http://farm3.staticflickr.com/2366/2527 ... 4e43_b.jpg
hilltop170
Posts: 3481
Joined: Sat May 06, 2006 6:05 pm

Re: Engine Compression Question

Post by hilltop170 »

Even if the cylinders walls, camshaft, and valves are good, the water traps in the oil sump are probably full of condensation and actively corroding the magnesium oil pan. There is not a good way to check for that corrosion. Maybe a really good mineral spirits flush of the pan and a good borescope thru the drain hole?
Richard Pulley
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
bagarre
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Re: Engine Compression Question

Post by bagarre »

When I bought 81d, she had only been run a few hours a year for last few years and was around mid time.
We changed the oil, ran it hard for 10 hours and changed it again. 250 hours later and no issues to speak of.
Every once in a while we have low compression on a few cylinders but only when checking it cold.

I'd say I still run it hard as I leave it at 2,550 on the dial which is about 2,600 with a tach checker.
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MoonlightVFR
Posts: 624
Joined: Sun Jan 02, 2005 5:55 pm

Re: Engine Compression Question

Post by MoonlightVFR »

Opportunity here

Please do a horoscope.video


share the video with us on this site


we hear about internal rust in engines but do you every see hard evidence? Cyl by Cyl video w numbers

Share the internal story

I think you may be looking at a solid airplane. Being back to active status will make you and the 170 community very happy.

Regards
gradyb, '54 B N2890C
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T. C. Downey
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Re: Engine Compression Question

Post by T. C. Downey »

pdb wrote:
Long storage times do not ruin the engine.
I agree when the engine has been properly pickled for storage but that's not really the issue here. This plane has only been run about 10 hours in 3 years and probably was not run long or hard enough to drive out the water vapor (combustion product) from the oil when being run on the ground.
How do you know that ?
My Engine sat from 1990 to 2013 and had virtually no corrosion, there was a slight haze on the cylinder walls that I could wipe outwit a rag.

Every engine is a study in its self, even the engine you are flying.
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pdb
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Re: Engine Compression Question

Post by pdb »

The issue of rust formation has been studied very well, especially with respect to multi-viscosity versus straight wgt oils, the latter of which has been shown to stick to cylinders much longer. The test for which BTW is rust formation on metal sheets dipped in oil then held vertically and monitored for rust formation. Not surprisingly, multi viscosity flows better at lower temperatures but runs off cylinder walls faster. The results indicate that if you use your engine a lot, multi is better in operation. If you don't fly much, single weight is better for rust protection while less active.

That said, there are so many variables for engines in storage. Were they properly prepared? What kind of oil, how long flown or not flown, dry interior continental climate or coastal swamp, heated hangar, etc, etc, etc. No matter how good your research, there are a lot of unknowns for any single engine.

So, like the man said, do a proper borescope, and if you like what you see, bid appropriately. Nothing is certain and you won't know about the oil pan until it leaks or the engine is torn down.
Pete Brown
Anchorage, Alaska
N4563C 1953 170B
http://farm3.staticflickr.com/2366/2527 ... 4e43_b.jpg
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ghostflyer
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Joined: Mon Apr 21, 2008 3:06 am

Re: Engine Compression Question

Post by ghostflyer »

All suggestions raised are very valid and should be done But what about the unseen ( unseeable) condition of the engine ? It takes a careful study of the log books to see the history of the engine and reading between the lines. For example have the cylinders been pulled and replaced? Cylinders are often abused in a engine and when they failed and they are often overhauled or repaired and tagged . CHT could have been at 500 degs for some time but after beads blasting it's nice shinny /clean and new valves its looks good as new . What's the average life of a cyclinder? There is no log book for a cylinder or hour meter. We had engines in aircraft that were doing RPT and doing very long flights and were run to manufactures specs run to TBO easily and the specs checked. Sometimes the engine was allowed under a approval run a extension of time . Most cylinders run to 4 overhauls and then the reliability factor showed up. However on GA aircraft we found 2 maybe 3 overhauls on cylinders you could get away with it but reliability factor often came in again . So the moral of this story is factor in the price of the engine (aircraft) when buying and yes cylinders do cost a arm and a leg but replacement with new is the best way to go . Plus when pulling a cylinder it allows a inspection of the other workings . PLUS I have been advising to owners of O- 300 engines to have their sump pulled and inspected(cleaned ) every 7 years or 700hrs . Or if the condition of the engine is unknown and logs books are sketchy The things that have been found in the bottom of sumps is beyond belief . Warning. The sludge in the bottom of a sump is very poison , it contains lead and other nasties so,wear gloves and dispose of the sludge thoughtfully .
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c170b53
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Re: Engine Compression Question

Post by c170b53 »

Top end repairs I wouldn't worry about but bottom end problems would be expensive, so it's the bottom I'd worry about. I think Tom got it right., in that each engine will have its story
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
Onewinglo
Posts: 114
Joined: Wed Sep 17, 2014 1:04 am

Re: Engine Compression Question

Post by Onewinglo »

Guys,
I see we have a lot to consider beyond just cylinder compressions when evaluating an engine. A few items I’ve gleaned are:
Total time SMOH?
Time STOH?
What kind of overhaul, field or repair facility? How well is the overhaul documented?
How many overhauls on the cylinders? Or how many total hours on the cylinders?
How often has it been recently flown? Sitting idle could lead to Internal Rust / Cylinder Wall Pitting, Cam Shaft Spalling.
Water / Sludge in the oil sump? Water leading to magnesium oil pan corrosion and sludge promoting accelerated wear.
Multi-viscosity or straight weight oil?
Complete engine logs?
My search for a 170, 170A or 170B is still on going. I’m a little better equipped in the search thanks to you guys!
JP White
C170A N1714D
My First Airplane!
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