Cylinder check

Why is every one insistent on using the wrong terminology for the aviation compression test?

http://www.enginebasics.com/Engine Basics Root Folder/Compression Test.html

we use the differential compression method. as given in both major manufacturers service bulletins.

no where in any of the engine manufacturers manuals does it say leak down test.

How can any intelligent conversation take place when either party doesn't know what the procedure is called or how it is completed?
 
Some one needs to explain the difference between a dynamic compression and static compression.

when that is understood, you will understand why 68/80 PSI is no problem.
 
Tom-D said:
Why is every one insistent on using the wrong terminology for the aviation compression test?

http://www.enginebasics.com/Engine Basics Root Folder/Compression Test.html

we use the differential compression method. as given in both major manufacturers service bulletins.

no where in any of the engine manufacturers manuals does it say leak down test.

How can any intelligent conversation take place when either party doesn't know what the procedure is called or how it is completed?
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Becuase whether someone tells me they have compression values of 70/80 60/80 75/80 etc,
or they tell me they have leak down results of same,
or if they tell me they have differential compression test results of same,

I know they are talking about the same thing.
Sometimes words matter. Sometimes, not so much.

I too get annoyed when someone says they are "loosing" when they mean "losing", but I know what they mean.
 
Kevin was the first to mention it that I saw, was he TEST done with a hot or cold engine? It can change the results.
 
You CAN Do a car type compression check on an airplane and its not a bad idea. One thing about that type of compression check, it is done with the engine spinning, not standing still. Ive seen leak down style compression checks where wiggleing the prop makes a signifigant difference. Things get "stuck" in there (lodged crooked?). I dunno for sure. Not a bad idea to do both, but no one does.
 
Tom-D said:
Why is every one insistent on using the wrong terminology for the aviation compression test?

http://www.enginebasics.com/Engine Basics Root Folder/Compression Test.html

we use the differential compression method. as given in both major manufacturers service bulletins.

no where in any of the engine manufacturers manuals does it say leak down test.

How can any intelligent conversation take place when either party doesn't know what the procedure is called or how it is completed?
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Well, maybe because an aircraft "compression" test doesn't involve any actual compression?

Leakdown is done at TDC on the compression stroke, and the piston is left there. Compression involves moving the piston from BDC to TDC with both valves closed, usually repeatedly.

Static compression is calculated. It's the ratio of cylinder volume at BDC to cylinder volume at TDC. There is a related "effective compression" which is tied to valve events. Neither has anything to do with engine diagnostics.

At least that's how it works everywhere where there is a such thing as spark advance.

Why aviation is different is beyond me. It's not like the engines work all that differently.

A related question is why aviation leak down is done at 80 PSI with reports in PSI, and automotive is done at 100 PSI with reports in percent (numerically equivalent, of course). Aviation pistons tend to be really big (a 360 CID car engine is a small block V8, not a 4banger), but that's not unheard of in big block cars.
 
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The differential compression test is a simple procedure to determine the condition of the sealing surfaces (valves and rings) and if there are any cracks in the cylinder head. Your mechanic uses this as a fundamental indication as to whether or not further inspection, testing or investigation is warranted.

A list of four numbers is, first of all, only half the story so posting them here is really only going to result in two pages of meaningless conjecture by every armchair mechanic on the internet. Why owners continue to do this is beyond me.

Why don't you just let your mechanic deal with it? If you have no faith in him then find one who you do trust.
 
comanche pilot said:
What about doing a hot leak down? I've heard people do it, I have never done it. I would think that cyl. is going to cool down pretty fast anyhow. Anyone with experience doing this and seeing a big difference between a cold and hot cyl? My I/A brought this up, when I had a weak cyl. We determined it was leaking by the rings, and just flew it. This cyl. has always been lower since factory new. We finally pulled the cyl. and found the compression ring gaps in alignment. I know they're supose to float and rotate but after reassembly and flying it seems to be back to the 68 psi it was before. :confused:
Been flying that way for the past 200 hrs. Blow-by and oil use doesn't seem excessive and the oil stays clean looking for the most part.
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Once rings seat, they don't really rotate much. Heck, even seating them up doesn't much either. That's why it's important to pay attention to proper ring placement on assembly.
 
Tom-D said:
Some one needs to explain the difference between a dynamic compression and static compression.

when that is understood, you will understand why 68/80 PSI is no problem.
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It is no problem if the differential is being lost at the rings. If it is hissing in the exhaust pipe, I would park the plane under a shade tree, pull the rocker cover and valve gear down to the bare stem and attach my drill motor to it, then pull the exhaust and reach up through with some lapping compound and proceed to lap the valve in place, saving me from a burned valve shortly down the road. But that's just shade tree mechanic me. I'll likely kill myself soon doing this kind of maintenance to keep my costs down.
 
Well I remember back in ought 5 when Wayne sent me out on the wing during flight and had me replace a cylinder. He had a heavy load and couldn't afford the downtime. But none of you young whippersnappers were around back then. Thouie!
 
coloradobluesky said:
Well I remember back in ought 5 when Wayne sent me out on the wing during flight and had me replace a cylinder. He had a heavy load and couldn't afford the downtime. But none of you young whippersnappers were around back then. Thouie!
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:lol: I worked for a guy who was crew chief/engineer on the B-36 and he would talk about exactly that! They could shut them down in flight and go out in the wing and fix them.
 
Henning said:
It is no problem if the differential is being lost at the rings. If it is hissing in the exhaust pipe, I would park the plane under a shade tree, pull the rocker cover and valve gear down to the bare stem and attach my drill motor to it, then pull the exhaust and reach up through with some lapping compound and proceed to lap the valve in place, saving me from a burned valve shortly down the road. But that's just shade tree mechanic me. I'll likely kill myself soon doing this kind of maintenance to keep my costs down.
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No need to pull the exhaust, just go in through a spark plug hole.
 
MAKG1 said:
Well, maybe because an aircraft "compression" test doesn't involve any actual compression?

Leakdown is done at TDC on the compression stroke, and the piston is left there. Compression involves moving the piston from BDC to TDC with both valves closed, usually repeatedly.

Static compression is calculated. It's the ratio of cylinder volume at BDC to cylinder volume at TDC. There is a related "effective compression" which is tied to valve events. Neither has anything to do with engine diagnostics.

At least that's how it works everywhere where there is a such thing as spark advance.

Why aviation is different is beyond me. It's not like the engines work all that differently.

A related question is why aviation leak down is done at 80 PSI with reports in PSI, and automotive is done at 100 PSI with reports in percent (numerically equivalent, of course). Aviation pistons tend to be really big (a 360 CID car engine is a small block V8, not a 4banger), but that's not unheard of in big block cars.
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READ

http://www.tcmlink.com/pdf2/sb03-3.pdf

No where is it called a leak down or a compression test.

Proper terms.. DIFFERENTIAL PRESSURE TEST
 
jesse said:
Tom, outside of aviation, the REST OF THE WORLD CALLS IT a leak down test....What is the problem?
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Most of the people in aviation who knows what it really is knows it isn't a leak down of any thing. It is a simple matter of showing a standard of the industry as a differential pressure test to determine where and how much leakage occurs under pressure.

Your Auto type of leak down test will not show you where the leak occurs it simply show there is no/or low compression.

Big difference, so use the proper terms when trying to explain what is happening
 
Tom-D said:
Most of the people in aviation who knows what it really is knows it isn't a leak down of any thing. It is a simple matter of showing a standard of the industry as a differential pressure test to determine where and how much leakage occurs under pressure.

Your Auto type of leak down test will not show you where the leak occurs it simply show there is no/or low compression.

Big difference, so use the proper terms when trying to explain what is happening
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Tom,
Now you have me confused. Can you help educate me on what is wrong with the term "leak down" test?
As I understand it, the cylinder is put under 80# pressure and a gauge measures how much the pressure drops(leaks down) over a certain time interval. What am I misunderstanding?
Thanks,
John
 
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Tom-D said:
Most of the people in aviation who knows what it really is knows it isn't a leak down of any thing. It is a simple matter of showing a standard of the industry as a differential pressure test to determine where and how much leakage occurs under pressure.

Your Auto type of leak down test will not show you where the leak occurs it simply show there is no/or low compression.

Big difference, so use the proper terms when trying to explain what is happening
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This is very much where you're confused tom. A cylinder leak town test in the automotive world is basically the same thing we do.

https://mobiloil.com/en/article/car-maintenance/car-maintenance-archive/how-to-do-a-leakdown-test

No different. Dump air in the cylinder. See how much leaks out. Listen to see where the leak is coming from. Make a decision as to if you want to investigate further.
 
JOhnH said:
Tom,
Now you have me confused. Can you help educate me on what is wrong with the term "leak down" test?
As I understand it, the cylinder is put under 80# pressure and a gauge measures how much the pressure drops(leaks down) over a certain time interval. What am I misunderstanding?
Thanks,
John
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The way I was taught to do a leak down test long ago on race car engines, you charged the cylinder at TDC, turned off the air supply, then timed the loss of pressure.:dunno: This was rarely performed and only when seeking the ultimate ring gap tune. Otherwise we just always used a dynamic compression test which gave us what we were looking for. The differential test we use on aircraft is similar to a leak down in that it is testing the cylinder seal integrity, however if you did a standard leak down test, the clearances would be so sloppy you couldn't get it to hold pressure long enough to get a significant result. Think of it like trying to get an accurate BP with a cuff drop spanning 1 second.
 
I'd say the reason it's done this way on "most" aircraft is because it's so convenient being able to position and hold with the propeller. You end up with more interpretable results as well as being safer and easier on the engine since you don't have to crank with the starter or spin the prop.

I say "most" because on Grobs and Diamonds with Rotax engines the manual specifically tells you to perform an automotive style dynamic compression test and what the values should read.
 
JOhnH said:
Tom,
Now you have me confused. Can you help educate me on what is wrong with the term "leak down" test?
As I understand it, the cylinder is put under 80# pressure and a gauge measures how much the pressure drops(leaks down) over a certain time interval. What am I misunderstanding?
Thanks,
John
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It's not a time-related test. On the tester, there are two gauges that measure the pressure on either side of a calibrated restriction which, for our little engines, is a hole .040" in diameter by .250" long with a 60° entrance angle. When there is pressure applied but no airflow through the restriction, the pressure will read the same on both sides of the restriction. As air starts to move through it, the restriction offers resistance so that a small airflow gives a slightly smaller reading on the second gauge and a large airflow gives a much smaller reading.

Leak_down_tester.GIF


For aircraft, we set the regulator to read 80 psi on the "standard pressure" gauge. If a cylinder is really tight, you might see 78 or 79 on the other gauge. Wiggling the prop a bit will often improve a reading by getting the rings to seat properly in their lands.

This job normally requires two people, one to hold the prop and the other to work the tester. That prop can get away on you if you turn it just a little too far away from TDC in trying to get a better reading. Imagine a big axe swinging around and trying to get you...

Dan
 
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jesse said:
This is very much where you're confused tom. A cylinder leak town test in the automotive world is basically the same thing we do.

https://mobiloil.com/en/article/car-maintenance/car-maintenance-archive/how-to-do-a-leakdown-test

No different. Dump air in the cylinder. See how much leaks out. Listen to see where the leak is coming from. Make a decision as to if you want to investigate further.
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Every auto mechanic I know screws a gauge in to the spark plug hole, cranks the engine, the cylinder pumps up to a pressure on the gauge.

What confuses me is simple, why people call a test some thing it is not.
 
Tom-D said:
Not when you position the prop correctly.
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But I (and most everybody else) often have to move the prop to get the best reading. Sometimes I've had to move as much as 20° away from TDC, on one side or the other, to get the piston and rings to seat. Big-bore engines (520s and 550s) can quickly develop enough torque to get away from you with that much movement. Smaller engines are no big deal.

If one just positions it at TDC and takes a reading without any movement, he risks getting falsely low readings.
 
Dan Thomas said:
But I (and most everybody else) often have to move the prop to get the best reading. Sometimes I've had to move as much as 20° away from TDC, on one side or the other, to get the piston and rings to seat. Big-bore engines (520s and 550s) can quickly develop enough torque to get away from you with that much movement. Smaller engines are no big deal.

If one just positions it at TDC and takes a reading without any movement, he risks getting falsely low readings.
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I used to do them all regularly by myself, wiggling the prop around and all. Now the dude across the ramp lost control of the prop in a DC-3 and that knocked the stand he was on down, but even a 1340 I do single person with no problem keeping the prop under control with a 80psi input through the standard orrifice gauge.
 
80 psi is used for safety reasons, you can normally handle that unless you are negligent and let it get away from you. Obviously, in normal operation the pressures inside the cylinder are far greater and the interval of pressure changes is very quick so even a very poor differential pressure test result such as say, 40/80 will most likely not even be noticed on a running engine.

This is essentially a "canary in the coal mine" test that will give very early indications of the breakdown or compromising of the valve and ring seals within the cylinder. People are always saying that a borescope test should supplement the compression test but in actuality by the time you are able to see anything significant with a borescope, let alone a $20 Chinese USB dental camera, you are going to have differential compression test results on the order of 0/80.
 
Silvaire said:
80 psi is used for safety reasons, you can normally handle that unless you are negligent and let it get away from you. Obviously, in normal operation the pressures inside the cylinder are far greater and the interval of pressure changes is very quick so even a very poor differential pressure test result such as say, 40/80 will most likely not even be noticed on a running engine.

This is essentially a "canary in the coal mine" test that will give very early indications of the breakdown or compromising of the valve and ring seals within the cylinder. People are always saying that a borescope test should supplement the compression test but in actuality by the time you are able to see anything significant with a borescope, let alone a $20 Chinese USB dental camera, you are going to have differential compression test results on the order of 0/80.
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Depends, you can pick up color/shading difference on the valve head and still be seeing compressions in the 50s and 60s. Usually it will have failed the ear to the exhaust test in the 60s as well if it was leaking at the valve.
 
Tom-D said:
Every auto mechanic I know screws a gauge in to the spark plug hole, cranks the engine, the cylinder pumps up to a pressure on the gauge.

What confuses me is simple, why people call a test some thing it is not.
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In those cases they don't call it a leak down check...not sure where you're disconnecting here.
 
jesse said:
In those cases they don't call it a leak down check...not sure where you're disconnecting here.
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I don't really give a rat's potoot, what any one does with their car. We were talking about what the proper nomenclature of what is commonly called a compression check in aviation. there are several here that refer to the compression check as a leak down , this is simply wrong, because the differential pressure test we preform is neither a compression check or a leak down check.

All we do is measure the difference across the orifice in the tester. That is all this does.
It does not create compression, It does not show how much air is leaking out of the cylinders. All we can determine is the air escaping the cylinder is some where between 0 and the amount of air entering the cylinder at 80 PSI.
listening for air escaping is only 1 of many trouble shooting procedure we use to determine the airworthiness of the engine/cylinder.
 
Henning said:
Depends, you can pick up color/shading difference on the valve head and still be seeing compressions in the 50s and 60s. Usually it will have failed the ear to the exhaust test in the 60s as well if it was leaking at the valve.
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Cylinders with exhaust valve rotators will have the same color over the entire valve head. These cylinders will have an eroded seat, and heat the whole valve equally.

The only way to see a bad seat is to remove the valve, then you can see the valve is most likely warped. Now you are into a cylinder rebuild or a new cylinder.
 
Henning said:
Depends, you can pick up color/shading difference on the valve head and still be seeing compressions in the 50s and 60s. Usually it will have failed the ear to the exhaust test in the 60s as well if it was leaking at the valve.
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Not in my experience. I've had cylinders with readings in the 30's that, off the engine in broad daylight with your face looking right down the bore you couldn't see a damn thing that would give you a clue there was anything wrong with it.

We're talking mils here folks, machine shop stuff. These aren't anomalies you can pick out with the naked eye. That's why a test you might perform would be to pour a cup of solvent into an inverted cylinder on the bench, go to lunch and then see if any had leaked out when you got back.

I know you've all seen the pictures on Savvy Maintenance webnars of discolored valves and such. First off, those pictures are right off the Continental Service Bulletin. Secondly, not a single one of them was taken with a USB cam or a Borescope. They are all of cylinders that had been removed from the engine because they already had a differential compression reading of ZERO.
 
Silvaire said:
Not in my experience. I've had cylinders with readings in the 30's that, off the engine in broad daylight with your face looking right down the bore you couldn't see a damn thing that would give you a clue there was anything wrong with it.

We're talking mils here folks, machine shop stuff. These aren't anomalies you can pick out with the naked eye. That's why a test you might perform would be to pour a cup of solvent into an inverted cylinder on the bench, go to lunch and then see if any had leaked out when you got back.

I know you've all seen the pictures on Savvy Maintenance webnars of discolored valves and such. First off, those pictures are right off the Continental Service Bulletin. Secondly, not a single one of them was taken with a USB cam or a Borescope. They are all of cylinders that had been removed from the engine because they already had a differential compression reading of ZERO.
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Amen. !!
 
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