question Acceptable Cylinder Compression - Lycoming O-235-C1

[sseeker]

Hi everyone,

 

Been a while since I've posted on here so hopefully putting this in the right place.

 

I'm in the process of looking to purchase an aeroplane (GA). Said aeroplane has a Lycoming O-235-C1 (115hp) and this would be its 3rd installation with the engine TT being 2500ish hours. There is a record of major repair/overhaul occurring at this point. The engine logbook now states it has 2000hrs remaining. This overhaul took place ~20 years ago so the 12 year calendar life has expired and the engine is now "on condition".

 

To my actual question, the cylinder pressures range from 60psi to 64psi over 80 which seems incredibly low. What is this saying about the engine? Lycoming service bulletins indicate 70+ psi is "acceptable" with a variance of not more than 5psi between cylinders. I'm not sure if this applies to their "low compression" engines.

 

While I'm interested in the aeroplane I won't waste my time getting a LAME pre-purchase done if the engine is shot or about to require a major overhaul. I'll continue my search.

 

This would be my first aircraft purchase and I'm no maintenance engineer, so all of this is new to me. If anyone is familiar with Lycoming engines or has some input to assist, I'd appreciate it.

 

Cheers,

Andrew

Edited May 30, 2020 by sseeker

[facthunter]

Get a borescope report on the surface of the bores. If there's evidence of much corrosion, because of sitting idle, they require replacing. That's the best way of fixing the problem, not reworking them. 2500 TT is not high but 2300 is the figure for TBO If I recall correctly. Mags are lifed also. Nev

[old man emu]

the cylinder pressures range from 60psi to 64psi over 80 which seems incredibly low

 

20% would be the the line where you would go pulling things apart to take a look, or use a borescope as Facthunter says.

 

Allowing an engine to operate "on condition" is a decision that only a LAME can make, and most won't allow an engine to run "on condition" unless they have been the only person servicing it for many years. The concession is also affected by how the aircraft is used. Approving "on condition" is a heavy weight for a LAME to bear.

 

If this is your first aircraft purchase, trust your gut feeling. Simply by posting your question you are showing a good deal of "buyer beware". You might be interested in the aircraft, but don't let your heart rule your head. There are plenty more birds in the sky for you.

[Flightrite]

See if you can find out its usage in recent times. I used to operate one in my Citabria. It was 41 years old with a TT of 1050 hrs (rare that's why I bought it) and comps where all around 70/80, used very little oil, ran like a Swiss watch. They are one of the most under stressed reliable engines Lyc made wth a TBO of 2400 hrs, you would have to go out of yr way to kill one. They are one of the few engines that have adjustable tappet clearances, it's a long shot but if the clearances are tight that won't help wth the compression testing, air escaping during the test via the oil fill tube means rings, via the exhaust means valves. Good luck, purchasing any plane has a certain amount of risk.

[sseeker]

Thanks for your input so far all.

 

See if you can find out its usage in recent times. I used to operate one in my Citabria. It was 41 years old with a TT of 1050 hrs (rare that's why I bought it) and comps where all around 70/80, used very little oil, ran like a Swiss watch. They are one of the most under stressed reliable engines Lyc made wth a TBO of 2400 hrs, you would have to go out of yr way to kill one. They are one of the few engines that have adjustable tappet clearances, it's a long shot but if the clearances are tight that won't help wth the compression testing, air escaping during the test via the oil fill tube means rings, via the exhaust means valves. Good luck, purchasing any plane has a certain amount of risk.

 

Since 2000 it's logged less than 100hrs on the engine. Last actual flight was ~12 months ago. 100hrly done back then.

 

I think I'm going to walk away from this one. Bargain price, but seems based on the compression figures, the engine could be on it's last legs. The notes on the overhaul indicate a few things were replaced but not the cylinders/re-boring or anything that would improve compression.

[onetrack]

The compression pressures are marginal - but they are not the best indicator of engine condition. I'd suggest a leakdown test is a better indicator of condition.

The O-235-C1 only runs a 6.75:1 compression ratio, but 60psi is regarded as the point where overhaul should be considered.

 

https://www.lycoming.com/sites/default/files/Cylinder%20Compression.pdf

[sseeker]

Thanks onetrack. That's what has me puzzled. Overhaul notes indicate the engine was stripped and everything was "within manufacturers limits" - yet the compression figures seem marginal after less than 100hrs of operation. The only parts replaced were the oil pump, spark plugs, various nuts and bolts + gaskets...

[Downunder]

100 hrs in 20 yrs?........ my alarm bells are ringing on that one!

 

Of course it's all relative to purchase price and proposed length of ownership.

An overhaul or repair factored in to a purchase price and amortised over your ownership period may provide very good value. Noting the safety factor of flying behind an overhauled or new engine.

An overhauled engine would give an increase in the outright value of the aircraft, so the money spent is not a total loss.

 

If looking for an overhaul quote, sending your engine overseas (airfreight) may provide better value than here, even with the poor exchange rate.

About Us - Aero Sport Power provide experimental engine overhauls and Pro Aero Aviation for certified engines.

I believe an engine returned after repair is exempt from gst but you'll have to check that.

[sseeker]

100 hrs in 20 yrs?........ my alarm bells are ringing on that one!

 

Of course it's all relative to purchase price and proposed length of ownership.

An overhaul or repair factored in to a purchase price and amortised over your ownership period may provide very good value. Noting the safety factor of flying behind an overhauled or new engine.

An overhauled engine would give an increase in the outright value of the aircraft, so the money spent is not a total loss.

 

If looking for an overhaul quote, sending your engine overseas (airfreight) may provide better value than here, even with the poor exchange rate.

About Us - Aero Sport Power provide experimental engine overhauls and Pro Aero Aviation for certified engines.

I believe an engine returned after repair is exempt from gst but you'll have to check that.

 

 

Thanks for that. It's amateur built and the builder lost his medical after completion. Pretty sure the hours were just test flights/periodic runs while stored.

 

Anyway - I've made the call to continue my search. I'm sure something better suited to me will come around soon

[IanR]

The compression or leak down number only say what each cylinder is like. The cost of replacement or overhauled cylinders is not huge.

However, on a little used engine the big problems will come from internal corrosion and deterioration on things like camshaft, etc.

When I bought my Auster it had done only 3 hours in the last year. One cylinder had 36 while the others were 78. It had corrosion and leaking round valve seat. This was fixed and the total was about $1500.

[Flightrite]

At the end of the day only you can make that choice? By the way what was the airframe the engine as attached too?

[old man emu]

By the way what was the airframe the engine as attached to?

 

I would like to know that too. You might be looking closely at the engine but there might be matters relating to the airframe that people here know about, but as a new entrant to the market, you might not.

 

If the O-235-C1 runs a 6.75:1 compression ratio, then the theoretical best compression reading you can expect is (6.75 x 14.7) = 99 psi at sea level. But that's not the value you are looking at with a leak-down test. That is a test of how well a cylinder can hold a given pressure. 80 psi is close enough to peak compression to give reliable assessment.

[440032]

What was the instrument used to obtain those low pressure readings? A seven times dropped thing from K-Mart, or something known to be accurate?

[facthunter]

With so little usage over a long period of time and without proper inhibiting it's going to have cylinder rusting for sure. I replaced mine and it's not cheap. There are lots of different cylinders on these motors, and I think the ones I had on the Citabria were one of the most expensive. The KIT comes with Pistons rings gudgeon pins valves all fitted ready to go . You would HAVE to inspect camshaft and followers also. I wouldn't even run that engine without removing the worst cylinder and inspecting the Camshaft. THAT is not an overly big job. IF you run it there's a fair chance you will do damage to it . They are a pretty bomb proof motor. Nev

[old man emu]

What was the instrument used to obtain those low pressure readings?

 

A little data, no matter how inaccurate, is better than no data at all.

 

In this case, if the el cheapo pressure gauge gave the values it did, then the results were useful - caveat emptor. Imagine if the shoddy gauge gave readings of 75 - 80 psi and the condition of the engine was not queried. We'd be soon reading about one of those dangerous amateur-built little planes plunging from the sky.

[Flightrite]

It wouldn't be hard to detect a diff reading down to 60 over 80, the air escaping would be very noticeable. You be seen engines that where idle for sometime slowly improve with usage as well.

[facthunter]

That's true , but I wouldn't suggest that in this instance. Nev

[onetrack]

Don't forget that even though the O-235-C1 has adjustable valve clearances, there's no screw adjusters on the rockers, as in most other engines.

 

The clearances are adjusted by purchasing longer or shorter pushrods - another added hidden cost, just to simply get correct valve clearances.

[Yenn]

There seems to be some difference of opinion here about what the quoted figures represent. They were posted as "60psi to 64psi over 80". That is just a bit of an odd way of quoting leak down pressures. Normally the "psi" is not used. They are numbers which are low but not catastrophically so. If I had the engine and it was runnable I would try again to see if the numbers improved, but if they are the best numbers that the seller could get, then there is a problem. It would need to be a good buy to make it worthwhile and Lycoming don't have a good reputation with camshaft corrosion in little used engines, also the age of the engine means that really the seals and other things that don't wear, but just deteriorate will need attention. I would guess that once you touched this engine it would attach itself to your back pocket.

[sseeker]

At the end of the day only you can make that choice? By the way what was the airframe the engine as attached too?

 

Zenair (Zenith) CH200. I'd joined several Zenith/Zenair groups on Facebook and Groups IO to get advice. Not many in Australia so the advice and resources in those groups was quite useful.

 

What was the instrument used to obtain those low pressure readings? A seven times dropped thing from K-Mart, or something known to be accurate?

 

It was performed by a LAME during a 100hrly - so I'd say a calibrated instrument.

 

It wouldn't be hard to detect a diff reading down to 60 over 80, the air escaping would be very noticeable. You be seen engines that where idle for sometime slowly improve with usage as well.

 

There was a note in the logbook that there were "nil defects" so I'm guessing he/she couldn't hear much.

 

There seems to be some difference of opinion here about what the quoted figures represent. They were posted as "60psi to 64psi over 80". That is just a bit of an odd way of quoting leak down pressures. Normally the "psi" is not used. They are numbers which are low but not catastrophically so. If I had the engine and it was runnable I would try again to see if the numbers improved, but if they are the best numbers that the seller could get, then there is a problem. It would need to be a good buy to make it worthwhile and Lycoming don't have a good reputation with camshaft corrosion in little used engines, also the age of the engine means that really the seals and other things that don't wear, but just deteriorate will need attention. I would guess that once you touched this engine it would attach itself to your back pocket.

 

Thanks Yenn. That's just the way I quoted it, the logbook quotes a figure per cylinder 1,2,3,4. I just gave a range because they all fall between 60 and 64psi. The Lycoming service bulletin outlining compression figures quotes PSI, so I assumed that's what the logbook figures were.

Edited May 31, 2020 by sseeker

[facthunter]

One track, I think that applies to hydraulic lifters to keep them in range of the self adjustment, surely.

In a motor that's been idle like that has, Leakdown figures and flying it become secondary considerations.. You MUST visibly check the amount of corrosion in these circumstances or you risk engine failure and damage... Its' NOT a &*#$@ lawnmower. Nev

[onetrack]

I presume it's this aircraft?

 

https://www.planesales.com.au/details/Listing/Single-Engine-Propeller/6041/2009-Zenith-CH200-Aircraft

[sseeker]

That's the one. I held off disclosing initially because I figured the seller would want some privacy. Ultimately anyone who looks at the aircraft is going to ask these same questions anyway, so not such a big deal.

[facthunter]

It does need to be sorted out re the zero timing and the condition it is in. Inhibiting engines is not just "if you feel like it"with GA and it applies (in principle) to all a/c engines. Nev

[onetrack]

Facthunter - Ahh, yes, my apologies, I misread the manual late last night and mixed up the hydraulic tappet and solid lifter engines.

The solid lifter O-235-C1 engines have adjustable screws for valve clearances and the hydraulic tappet models are the ones requiring pushrod changes for valve clearance adjustment.

 

https://www.lycoming.com/sites/default/files/O-235%26O-290%20Operator%20Manual%2060297-9.pdf