Differential compression test
Textron Lycoming will attempt to repeat the operating instructions which accompany the equipment. This should be read and followed carefully for best results. The following recommendations will supplement the instructions accompanying the equipment:
1. A standard 80 lbs. of input air is recommended. More pressure makes it difficult to hold the prop.
2. A loss in excess of 25 percent of the 80 lbs. or a reading of 60/80 is the recommended maximum allowable loss.
3. The engine should have been run up to normal operating temperatures immediately preceding the compression check. In other words, we recommend a hot engine check.
4. The differential compression equipment must be kept clean and should be checked regularly for accuracy. Check equipment with the shutoff valve closed and regulated pressure at 80 psi (the cylinder pressure gauge must indicate 80 psi plus or minus 2 psi) and hold this reading for at least 5 seconds. Homemade equipment should be carefully calibrated.
5. Combustion chambers with five piston rings tend to seal better than three or four piston rings, with the result that the differential check does not consistently show excessive wear or breakage where five piston rings are involved.
6. If erratic readings are observed on the equipment, inspect compressor system for water or dirt.
7. If low readings result, do not remove the cylinders without a recheck after running up the engine at least three minutes, and refer to the cross checks listed later.
8. If valves show continual leakage after recheck, remove rocker box cover and place a fiber drift on the rocker arm immediately over the valve stem and tap the drift several times with a 1- or 2-lb. hammer. When tapping valves thusly, rotate the prop so that the piston will not be on top dead center. This is necessary in some engines to prevent the valve from striking the head of the piston. Then rotate engine with the starter and recheck compression.
9. Caution. Take all necessary precautions against accidental firing of the engines.
Cross checking is important
Rather than rely on one source of information concerning the condition of the combustion chamber, it is wise to make cross checks, particularly when the compression readings are questionable. Therefore, we would like to recommend the following before removing a cylinder.
1. Remember that spark plugs tell a story. Carefully check the spark plugs removed from any cylinder with a low reading.
2. Use at least a goose-neck light or preferably a borescope and carefully check the top of the piston and cylinder walls.
3. Consider the health history of the engine. Has it had previous difficulty of this nature?
4. Has the pilot observed any loss of power in the engine during flight or during run up?
5. How has the engine been maintained and operated during its life? If the maintenance and care have been proper and consistent, there is less likelihood of trouble.
6. The supervisor of maintenance should evaluate the known factors such as those discussed here and make a recommendation to the pilot.
Whatever your opinion of the compression check as a maintenance aid, it is probable that no pilot or mechanic would care to omit it during a 100-hour or annual inspection. On the other hand, since most everyone seems to use it on the flat opposed engines, we ought to share our experiences with its application to our powerplants. This has indicated that the differential is the best method of checking compression, and particularly so when the readings are charted as a trend over a number of routine inspections. It is a good tool for preventive maintenance and aids in avoiding in-flight failures. Cross checking is good procedure, rather than relying on one source of information concerning the condition of the combustion chamber.
How they work — what to look for
There are a variety of differential compression testers on the marketplace today. For the most part, the devices are fairly simple. The specifications for how they are suppose to be manufactured are called out in AC 43.13-1B, so it's not a secret as to what it takes to put one of these together. Yet, if you begin to shop around, you find a wide variety of prices (from $35 to over $100). So what do you purchase and what's the difference?
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