Excessive crankcase blow-by is most likely caused by wear in which piston ring set?

• A. Worn oil control rings.
• B. Excessive engine speed.
• C. Worn compression rings.
• D. A plugged crankcase filter.

Answer: (C)

Crankcase blow-by happens when some of the high-pressure combustion gases escape past the piston rings into the crankcase. The part of the ring set that primarily controls this leakage is the compression rings. When these rings wear, their ability to seal the combustion chamber diminishes, so more gas bypasses the rings and enters the crankcase, causing blow-by.

Worn oil-control rings mainly affect oil management—letting more oil slip into the combustion chamber and burn, which is related to oil consumption and smoke, not the primary gas leakage into the crankcase. A plugged crankcase filter can cause crankcase pressure issues, but it doesn’t create the seal failure that lets combustion gases flood the crankcase. High engine speed on its own isn’t the root cause; it can worsen symptoms if the rings are already worn, but the wear in the compression rings is the main driver of excessive blow-by.