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Home> Aircraft Maintenance Articles Q. Is there any way to correct cylinder blow-by other than pulling the cylinder and replacing?
A. Blow-by is the passage of hot combustion gas past the piston ring belt and into the crankcase. The problem is usually found by the mechanic when he does a "compression" test. It is usually described as a compression loss past the rings. Blow-by is the symptom produced when you have a worn cylinder barrel, worn piston rings, worn piston ring lands, or poor ring to barrel seating. These things are hard to correct without removing the cylinder and doing some repair work. Sometimes people have tried a different oil, or a different oil viscosity with some temporary success. Always a motivating factor in fixing a problem is knowing what might happen if it is not fixed. Exhaust gases at the EGT probe run about 1,400 degrees F. Inside the cylinder, combustion gases runs closer to 4000 degrees F. The top of the piston is a large heat absorption surface that would quickly melt if it couldn't conduct heat to the cylinder. The piston's heat conduction path is primarily to the piston rings and there to the cylinder barrel, cylinder barrel fins, and into the atmosphere.
The heat path requires a good contact surface between the rings and cylinder wall. A good contact surface also makes a good sealing surface where combustion gases can't leak. Blow-by past the rings means they aren't making good contact and can't conduct heat out of the piston. Consequently, the piston gets hot and starts cooking the oil in the piston rings. Next, the rings stick, once stick you have massive blow-by. Stuck rings have a habit of breaking and then scoring your cylinder wall. Everything gets expensive real quick.
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