lml@lancaironline.net Mailing List Archive

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.......

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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