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: 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: 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. 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.  10-40 Differential Pressure Tester 10 15 20 25 30 35 40 45 50 55 60 65 70 75 10 15 20 25 30 35 40 45 50 55 60 65 70 75 Regulated pressure gauge Shutoff valve To air compressor Piston on true top dead center Cylinder pressure gauge Metering orifice Figure 10-53.  Differential compression tester diagrams.  Figure 10-54.  Compression tester and adapter.  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 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, 