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