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## Cylinder Compression Tests 

The cylinder compression test determines if the valves, piston rings, and pistons are adequately sealing the combustion chamber. If pressure leakage is excessive, the cylinder cannot develop its full power. The purpose of testing cylinder compression is to determine whether cylinder replacement is necessary. The detection and replacement of defective cylinders prevents a complete engine change because of cylinder failure. It is essential that cylinder compression tests be made periodically. Low compression, for the most part, can be traced to leaky valves. 

Conditions that affect engine compression are: 

1. Incorrect valve clearances 

2. Worn, scuffed, or damaged piston 

3. Excessive wear of piston rings and cylinder walls 

4. Burned or warped valves 

5. Carbon particles between the face and the seat of the valve or valves 

6. Early or late valve timing 

Perform a compression test as soon as possible after the engine is shut down so that piston rings, cylinder walls, and other parts are still freshly lubricated. However, it is not necessary to operate the engine prior to accomplishing compression checks during engine buildup or on individually replaced cylinders. In such cases, before making the test, spray a small quantity of lubricating oil into the cylinder(s), and turn the engine over several times to seal the piston and rings in the cylinder barrel. 

Be sure that the ignition switch is in the OFF position so that there is no accidental firing of the engine. Remove necessary cowling and the most accessible spark plug from each cylinder. When removing the spark plugs, identify them to coincide with the cylinder. Close examination of the plugs aid in diagnosing problems within the cylinder. Review the maintenance records of the engine being tested. Records of previous compression checks help in determining progressive wear conditions and in establishing the necessary maintenance actions. 

## Differential Pressure Tester 

The differential pressure tester checks the compression of aircraft engines by measuring the leakage through the cylinders. The design of this compression tester is such that minute valve leakages can be detected, making possible the replacement of cylinders where valve burning is starting. 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 the orifice results. 

As the airflow and pressure changes, pressure varies accordingly in the same direction. If air is supplied under pressure to the cylinder with both intake and exhaust valves closed, the amount of air that leaks by the valves or piston rings indicates their condition; the perfect cylinder would have no leakage. The differential pressure tester requires the application of air pressure to the cylinder being tested with the piston at top-center compression stroke. [Figure 10-53] 

Guidelines for performing a differential compression test are: 

1. Perform the compression test as soon as possible after engine shutdown to provide uniform lubrication of cylinder walls and rings. 

2. Remove the most accessible spark plug from the cylinder, or cylinders, and install a spark plug adapter in the spark plug insert. 

3. Connect the compression tester assembly to a 100 to 150 psi compressed air supply. [Figure 10-54] With the shutoff valve on the compression tester closed, 

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