Posted for "Frederick
Moreno" <frederickmoreno@bigpond.com>:
[Please see Teledyne
Continental Web site, service bulletins, SB03-3
http://www.tcmlink.com/pdf2/SB03-3.pdf referenced as "attached
Continental service bulletin" in the following
post.]
I also accept the challenge posted by
Swain.
Experience is wonderful, but factory
recommendations based on fleet wide
experience are even
better. Grayhawk's contribution below is
good. I
suggest that the attached
Continental service bulletin is even better for
those of
us driving big bore Continentals.
Short message is:
leakage tests too frequently lead to pulling
jugs
unnecessarily. Mechanics love it because it is a
great revenue generator.
But the reality is that readings vary a lot
from test to test due to a
variety of reasons, and the uncertainty in
any given reading is larger than
many appreciate. A low
reading should result in more investigation, not
automatic jug
removal.
For big bore Continentals, the factory suggests using
the appropriate big
bore tester with a reference
orifice. (See attached.) The reference
orifice
takes into account effects of temperature and altitude, and
the
orifice is sized for bores greater than five
inches. The drill is to
connect to air supply, close flow
to cylinder, open flow to reference
orifice, adjust inlet pressure to
80 psi, and then read the orifice
pressure which is the
reference p essure for future comparisons.
The reference pressure is used
to decide if it is necessary to pull a
jug
Maybe.
If the
cylinder tests at or below the reference pressure, you
may have a
problem. Otherwise, continue operation based on
continung on condition
evaluation, and consider a boroscope
examination for further data. Now we
can get cheap video
cameras that will fit in the spark plug hole, so
boroscope
inspections have gone from expensive to cheap and easy.
Now, for the shocker. I bought the goodies and
tested my IO-550. The
reference pressure I measured
(and re-measured again on subsequent tests)
is 42
psig. You read right. 42, four two,
forty-two. So the Continental
recommendation is if you do
a leakage test properly (hot engine, correct
procedure, calibrated
instruments) and if you get 70 or 60 or 50 (that is,
above 42 for my
test conditions), continue operating the engine, and test
again to
see if the reading is changing.
Traditional A and P's hate
this suggestion. It flies in the face of the
traditions developed
fifty years ago. But this is 2012 and there are a lot
more
data available. The procedure outlined by Continental is what
is
known as "best known method" today. Tomorrow something
better may surface.
But today's best known method trumps
yesterday's traditions. TIme marches
on.
Leakage tests are useful, but interpretation and
subsequent action need to
be made thoughtfully, based on
investigation and data, and not opinion.
Do it
using the tools the fighter pilots use (and now many in industry
use) - OODA. Observe, Orient (your thinking), Decide,
Act.
There are three steps before acting.
With all thy getting, get thee
understanding.
Fred
Moreno
-------Original
Message-------
From: Sky2high@aol.com
Date: 28/01/2012
2:40:37 AM
To: lml@lancaironline.net
Subject: Re: [LML] Re:
Lycoming exhaust valve questions
Grayhawks says high
readings on a Lyc 320,360 may be OK based on the tester
orifice.
Here's a bit of additional info:
FAASTeam Maintenance
Safety Tip
November 2011
Differential Compression Test (AC
43.13-1B)
The differential pressure tester is designed to
check the compression of
aircraft engines by measuring the leakage
through the cylinders caused by
worn or damaged
components.
The operation of the compression tester is based on the
principle that, for
any given airflow through a fixed orifice, a
constant pressure drop across
that orifice will result. The
restrictor orifice dimensions in the
differential pressure tester
should be sized for the particular engine as
follows:
(Although AC 43.13-1B was revised in 1998, the following
information was
revised in Chg. 1 in September 2001. The
identification criteria was
revised
from using “engines with
a certain cubic inch displacement” to
cylinder
bore)
(1) For an engine cylinder having
less than a 5.00-inch bore; 0.040-inch
orifice diameter;
.250 inch long; and a 60-degree approach angle.
(2) For an
engine cylinder with 5.00 inch bore and over: 0.060 inch
orifice
diameter, .250 inch long, and a 60 degree approach angle.
Remember, some of the smaller production engines have 5
inch or larger
cylinder bores (e.g. Lycoming O-320-A1A has a cylinder
bore of 5.125
inches)
Therefore, an orifice of 0.060
diameter should be used to perform the
compression test.
See
AC 43.13-1B, Chapter 8, paragraph 8-14 for more information.
ALSO at
Sacramento Sky Ranch note that Lyc's are expected to result in
higher
pressure readings - BUT orifice is not
mentioned.......
================
In a message
dated 1/27/2012 10:24:58 A.M. Central Standard Time,
casey
gary@yahoo.com writes:
Okay, I'll take the dare from
Swain and post what might be the more "normal
reply
:-). I think he is being a little of a perfectionist. I
don't have
his credentials, but from what I have seen, a compression
leakage of 74 to
76 is the most common. A compression
number up to 78 suggests that the
engine is pumping excess oil past
the rings and the oil is sealing
that
leak
path. So I would worry more about the
ones that measure 78 than I would
about the one that is at
75. Assuming you check the compression every 100
hours or
so, I would wait until you have more than 2 compression tests
with
ever-dropping values before I would even start to
worry. That will take
another 200 hours of
operation. I wouldn't worry until the compression
dropped
below 70. And like Dan said, it is always a good idea to warm
the
engine and then do the compression test as soon as possible
afterward. I
ve
gone as far as to warm the engine
with the cowl off so I could do it faster
but to just idle
on the ground is not a good thing just before
a
compression
test. If I were really into the
diagnostics I would warm the engine (cowl
off) at the normal runup
rpm and then run it at full power for maybe 15
seconds, pulling the
mixture while at full power to get a "clean" shut-off.
Then
you'll get as good a test as possible.
And why do
you change oil every 25-35 hours? With a modern engine
and
modern oil, I don't see any reason to change oil that
often. I would run
it
at least 50 hours and try
to change it before 75. If the engine is run
often, even
100 hours isn't out of line. What does the oil look like
at
the
oil change? Can you see the dipstick
through the oil? If so, it probably
doesn't need to be
changed yet.
Gary
Previously, from
Swain:
Hi Dan,
My opinion will differ from most here. It is
based on 28 years as a
professional aircraft mechanic and a cylinder
overhaul shop owner for 17
years. Lycoming says that NO static
leakage is allowed during a compression
test and I agree. What will
happen is that if your ex. valve continues to
leak past it's seat it
will errode a gas path in the seat and valve face.
This will cause a
"hot spot" in that area that can eventually lead to
valve
face breakage which might do major damage to one or
more cyl. I say more
than one because I have personally seen a piece
of ex. valve leave it's cyl
through it's intake port and
enter the next cyl. beside it and lodge itself
into the face of that
piston while bending the other cyl. intake valve.
(BTW
I
keep that piece of valve in my desk drawer as a reminder of what
can
happen). The only way a valve can cool itself is through the time
it sits
on
it's valve seat to transfer heat from the valve
to the head and also
through
the valve stem. That's why when
guides get worn and seats leak you wear out
valve stems and guides
much quicker, there is not as much metal contact for
cooling and the
psi of force on the wear surfaces increases (same
force
less
wear surface).
Remember one compression
test is only a snapshot of what is happening in
your engine. You need
more than one and other information to develope a
trend on what your
engine is really doing.
I would go fly your airplane at high power
settings for one hour and come
back and take another compression
test. Then you might try some of the
tricks mentioned here ( I have
never had much luck with short cuts). If it
was my airplane I would
pull the cyl. and fix the problem and not screw
around with wobble
test, staking the valve, or trying to lap a
valve
without
pulling the cyl. A gasket set is less than $20
bucks and you don't have to
replace the rings if there are
serviceable and you don't hone the cyl..
I have repaired/overhauled
over a 1000 Continental and Lycoming cylinders
and it does not take
very long to R&R a cyl.
Swaid
Rahn
A&P,I.A.,ATP
LIVP-T(Walter)
LIVP-T(Garrett
under construction)
On Tue, Jan 24,
2012 at 11:06 AM, Dan & Kari Olsen
<olsen25@comcast.net>
wrote:
LML
Gang,
Doing the annual on my 320 this
week. Compression check shows 77-78 on
cylinders 1, 2,
4. #3 is 75 and I hear pretty good leakage through
the
exhaust, indicating a leaking exhaust valve. I’ve had
no indication of
sticking valves or morning sickness. I
pulled the rocker cover and there
is
no abnormality
there. This engine is typically run lean of peak at
power
settings of 55-65% power. Oil changes every
25-35hrs. Engine has 623hrs
since
new.
My question to those of you who are much more
experienced with the Lycoming
4-cylinder engines, is what should be
my next course of
action?
· Do the
valve lapping procedure per the Lyc service
instruction?
· Start
using TCP and re-check in a few
hours?
· Go run the
engine hard and re-check the
compression?
·
Something else?
I really want to nip this in the bud and
not end up with a burned exhaust
valve, requiring pulling a
jug.
Appreciate your thoughts and
responses.
Cheers!
Dan
Olsen