Gentlemen, My post to Dan is not a dare or a challenge here on this forum, it is merely trying to pass on the information that I would give to my other customers. Notice I recommended to run/fly the aircraft and take another reading before pulling the cyl. and that a compression test is not all inclusive but you need other information to properly analyze the condition of your engine/cyl. I am glad someone posted some service information here as I don't like to work from memory alone or paraphrase tech. info because that leads to errors. Because I am an aircraft owner, pilot and mechanic I have a three sided point of view. Dan said he had a leaking exhaust valve. If it does not "clear up" after a one hour maintenance flight it probably won't after a 2,3,4, or 20hr. test flight but I would check it a few times to be sure. Because I know that Lycoming exhaust valves are sodium filled 1/2 diameter shanked valves and assembled with .005" clearance when new and are allowed to wear .001" per 100 hours after 300 initial hours excessive heat will destroy the valve guide very quickly as the table of limits allow up to a total of .018" guide to stem clearance as the service max( which is really loose). If the valve is leaking at the seat it cannot transfer the stored heat to the valve seat/head which will accelerate the wear on the valve stem.
My concerns: Owner- I don't want to spend anymore money than I have to to keep my aircraft airworthy. Ex valves alone are over $230 each. Pilot- I don't want this engine to drop a valve and cause me to have to get it repaired on a cross country flight at a shop I don't know, rental car home & back, or worse.
Mechanic- I don't want to send out a cyl. for a guide replacement, seat grind or weld repair while tieing up my hangar space, extra down time, having to box and ship, phone repair shop, float expenses on my credit card when I could have fixed it by lapping the valve on the bench for 15 min. and a $20 gasket set.
There are many good reasons to fix things early before they become real problems. Also my wife and family like it better when we fly somewhere and can come back on our own schedule and not the airplanes. My wife and family do not have unconditional love for airplanes like most mechanics I have met they just expect flawless performance 100% of the time. Most mechanics like myself understand that when proper care and maintenance is not performed on aircraft the aircraft will breakdown and wear out sooner than they should. Short cuts and differed maintenance usually lead to larger repairs and more money and more down time and nobody wants that.
And contrary to some beliefs here most mechanics don't "love it because it is a great revenue generator" about pulling cylinders. It ties up hangar space and screws up schedules for all involved. I don't like to see customers have to endure large unscheduled maintenance expenses just as I would not want to either.
Quote-"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." The Traditional A&P'S that I know and work with usually spend a good bit of time reading the manufacturers service information, after all it is required by FAR to do so. And the service bulletin referred to above is a revision to the one that came out around 1996. A older mechanic at my competitors shop told me about the master orifice and helped me calibrate my compression tester in 1996. He got his A&P license in the 1940's. BTW he is not in it "just for the money "either. Just like me most of his customers are his personal friends also.
Most mechanics I know take real pride in their profession and are always learning and trying to improve themselves, like most pilots I know. So here is the question. Which is more noble? An A&P mechanic who loves aircraft so much that he wants to learn to fly them so he can understand them more OR a pilot who loves aircraft so much that he wants to learn to work on them or build one so he can understand them more?
I think at the end of the day maybe we are just all aviators. Respectfully, Swaid Rahn
On Sun, Jan 29, 2012 at 8:47 PM, John Hafen <j.hafen@comcast.net> wrote:
Thanks Swain.
Very Helpful. On Jan 29, 2012, at 2:16 PM, Colyn Case wrote: I think Swain's main point is you don't want to let the exhaust valve go if it's starting to get a hot spot. Leaks are associated with bad seal at the valve. maybe worry more about the bore scope than the compression reading?
On Jan 27, 2012, at 11:06 AM, Gary Casey wrote:
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
--
Swaid L. Rahn
Indigo Aviation, Inc.
940 Mock Road
Springfield, Ga. 31329
Cell 912.655.0966
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