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AC 43.13-1B 9/8/98 (a) Perform the compression test as soon as possible after the engine is shut down to ensure that the piston rings, cylinder walls, and other engine parts are well-lubricated. (b) Remove the most accessible spark plug from each cylinder. (c) With the air valve closed, apply an external source of clean air (approximately 100 to 120 psi) to the tester. (d) Install an adapter in the spark plug bushing and connect the compression tester to the cylinder. (e) Adjust the pressure regulator to obtain a reading of 20 psi on the regulator pressure gauge. At this time, the cylinder pressure gauge should also register 20 psi. (f) Turn the crankshaft, by hand, in the direction of rotation until the piston (in the cylinder being checked) is coming up on its compression stroke. Slowly open the air valve and pressurize the cylinder to 80 psi. CAUTION: Care must be exercised in opening the air valve since suffi- cient air pressure will have built up in the cylinder to cause it to rapidly ro- tate the propeller if the piston is not at top dead center (TDC). (g) Continue rotating the engine against this pressure until the piston reaches TDC. Reaching TDC is indicated by a flat spot or sudden decrease in force required to turn the crankshaft. If the crankshaft is rotated too far, back up at least one-half revolution and start over again to eliminate the effect of backlash in the valve operating mechanism and to keep piston rings seated on the lower ring lands. (h) Open the air valve completely. Check the regulated pressure and readjust, if necessary, to read 80 psi. (i) Observe the pressure indication of the cylinder pressure gauge. The difference between this pressure and the pressure shown by the regulator pressure gauge is the amount of leakage through the cylinder. A loss in ex- cess of 25 percent of the input air pressure is cause to suspect the cylinder of being defec- tive; however, recheck the readings after oper- ating the engine for at least 3 minutes to allow for sealing of the rings with oil. (j) If leakage is still occurring after a recheck, it may be possible to correct a low reading. This is accomplished by placing a fi- ber drift on the rocker arm directly over the valve stem and tapping the drift several times with a hammer to dislodge any foreign mate- rial between the valve face and seat. NOTE: When correcting a low read- ing in this manner, rotate the propel- ler so the piston will not be at TDC. This is necessary to prevent the valve from striking the top of the piston in some engines. Rotate the engine be- fore rechecking compression to reseat the valves in the normal manner. 8-15. SPARK PLUGS. The spark plug pro- vides the high-voltage electrical spark to ignite the fuel/air mixture in the cylinder. The types of spark plugs used in different engines will vary with regard to heat range, reach, thread size, and other characteristics required by the particular installation. a. Heat Range. The heat range of a spark plug is the principal factor governing aircraft performance under various service conditions. The term “heat range” refers to the Page 8-8 Par 8-14PDF Image | AFS-640
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