Compression readings

It just appears to me that there is a mentality that the top end or
bottom end is a single entity.
When one part needs work it's better to replace the whole part.
As far as I'm concerned, it's generally better to jusr fix the offending
problem and then continue on.
Why would anybody want to do a total OH when there is one cylinder that
needs attention, regardless of TSMOH?




Newps wrote:


wrote:

First of all, thank you for answering the question.

Secondly, the first answers were not serious because it was obvious
from the
answers that they did understand the question, and chose not to answer
it.
There were some pretty detailed answers but none gave at what reading
would
one start considering the cylinder a problem.


You asked at what compression reading would you decide to overhaul the
engine. The answer is compression is just one factor. I had a cylinder
at one annual blow a 20/80. Doing an overhaul never entered my mind.
That cylinder turned out to be cracked. So you replace the cylinder and
move on. It's really only when your engine starts to make metal that
you contemplate overhaul. Although I had my engine make metal once,
turned out to be the shaft in my starter adapter. Replace the adapter
and the metal went away.

Secondly, the first answers were not serious because it was obvious from
the
answers that they did understand the question, and chose not to answer it.
There were some pretty detailed answers but none gave at what reading
would
one start considering the cylinder a problem.

That's not the question you asked. You asked what compression readings
should one consider an engine overhaul. That's like asking what blood
pressure should trigger a heart transplant.

The general rule of thumb is that any cylinder below 70 should be monitored
more closely and anything below 60 should be looked at for signs of
distress. But deciding on an overhaul of this cylinder, all the cylinders,
or the entire engine would depend on wide variety of factors.

- Mark

In article M0cvd.751595$8_6.347661@attbi_s04,
"markjen" wrote:

Secondly, the first answers were not serious because it was obvious from
the
answers that they did understand the question, and chose not to answer it.
There were some pretty detailed answers but none gave at what reading
would
one start considering the cylinder a problem.

That's not the question you asked. You asked what compression readings
should one consider an engine overhaul. That's like asking what blood
pressure should trigger a heart transplant.

The general rule of thumb is that any cylinder below 70 should be monitored
more closely and anything below 60 should be looked at for signs of
distress. But deciding on an overhaul of this cylinder, all the cylinders,
or the entire engine would depend on wide variety of factors.

- Mark

You also want to know the *quality* of the low compression reading. you
have to listen for the air leakage.

Open the oil cap and listen for air hissing into the crankcase. If so,
there is ring blowby, which may happen just because the ring ends are
lined up.

Listen to the exhaust pipe. Air escaping there indicates leaks at the
exhaust valve -- perhaps just a piece of crud temporarily under the
exhaust valve, or a burned valve or a poorly-seated valve.

Listen to the intake. Air escaping there indicates a leak at the intake
valve -- same causes as exhaust valve leaks.

Listen for air escaping around the cylinder head. Also, wet a finger and
pass it around the head and feel for any breezes. If you detect leaks
here, you have a cracked cylinder head and will have to replace it.

Places to look for cracks: around the sparkplug holes, between the
valves, around the exhaust galleys.

For WIW these are the compression readings from our O-320E2D Lycoming
which has 4 cyls of the same bore but less stroke as your 540. The
data is from 1977 thru 1998.

Cyl 1
70/72/67/66/71/71/70/74/72/68/72/72/72/72/68/73/73/73/74/71/66/80
(1977/....1998__)
Cyl 2
70/73/70/66/73/72/68/74/70/68/66/71/71/67/72/70/7271/71/71/80

Cyl 3
70/74/75/68/71/68/70/75/70/66/68/62/71/61/71/75/75/74/66/64/80

Cyl 4
71/70/70/66/70/69/67/70/67/68/66/72/69/71/64/70/70/73/68/70/80

As you can see sometimes some were up.... and sometimes some were down.
At 1680 hrs TTSN now, we still have never had a cyl off. Oil
consumption is still 30+ hrs/qt. When one was low or marginal, we'd
check it again in maybe 10 hrs and would usually find it up again. If
you plot it, there is a slight long term reduction in average
compression though. All readings were with the same gage (That can be
a big variable too).

Compression data since then isn't on this computer, but it hasn't
really changed its characteristics. I have the data elsewhere.

John Thorpe also once said that taking the upper spark plug out before
doing a compression test will cause debris to fall on any open valve,
giving a false low reading.

Cyl 1
70/72/67/66/71/71/70/74/72/68/72/72/72/72/68/73/73/73/74/71/66/80
(1977/....1998__)
Cyl 2
70/73/70/66/73/72/68/74/70/68/66/71/71/67/72/70/7271/71/71/80

Cyl 3
70/74/75/68/71/68/70/75/70/66/68/62/71/61/71/75/75/74/66/64/80

Cyl 4
71/70/70/66/70/69/67/70/67/68/66/72/69/71/64/70/70/73/68/70/80

You didn't find it all suspicious that the last (1998) readings were all
*perfect*?

Personally, I'd be checking my procedures or my gauge. Ain't no way those
cylinders all jumped from merely okay to all "80"s -- especially since they
were NEVER "80"s, even in back in '77.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:
Cyl 1
70/72/67/66/71/71/70/74/72/68/72/72/72/72/68/73/73/73/74/71/66/80
(1977/....1998__)
Cyl 2
70/73/70/66/73/72/68/74/70/68/66/71/71/67/72/70/7271/71/71/80

Cyl 3
70/74/75/68/71/68/70/75/70/66/68/62/71/61/71/75/75/74/66/64/80

Cyl 4
71/70/70/66/70/69/67/70/67/68/66/72/69/71/64/70/70/73/68/70/80


You didn't find it all suspicious that the last (1998) readings were all
*perfect*?

Personally, I'd be checking my procedures or my gauge. Ain't no way those
cylinders all jumped from merely okay to all "80"s -- especially since they
were NEVER "80"s, even in back in '77.

Geez Jay, thats the reference pressure...

Darrel Toepfer wrote:

Jay Honeck wrote:
Cyl 1
70/72/67/66/71/71/70/74/72/68/72/72/72/72/68/73/73/73/74/71/66/80
(1977/....1998__)
Cyl 2
70/73/70/66/73/72/68/74/70/68/66/71/71/67/72/70/7271/71/71/80

Cyl 3
70/74/75/68/71/68/70/75/70/66/68/62/71/61/71/75/75/74/66/64/80

Cyl 4
71/70/70/66/70/69/67/70/67/68/66/72/69/71/64/70/70/73/68/70/80


You didn't find it all suspicious that the last (1998) readings were all
*perfect*?

Personally, I'd be checking my procedures or my gauge. Ain't no way those
cylinders all jumped from merely okay to all "80"s -- especially since they
were NEVER "80"s, even in back in '77.

Geez Jay, thats the reference pressure...

If it is, then there aren't enough readings for the years 1977 to 1998.

George Patterson
The desire for safety stands against every great and noble enterprise.

If it is, then there aren't enough readings for the years 1977 to
1998.

The annuals sometimes took advantage of the calendar year effect, so
they tended to be slightly more than a year apart. Hence there ended
up being an apparent year missed.
The last listing was the common reference pressure.

The annuals sometimes took advantage of the calendar year effect, so
they tended to be slightly more than a year apart. Hence there ended
up being an apparent year missed.
The last listing was the common reference pressure.

Ah -- that's what threw me.

Thanks for the clarification...
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"