the leakage of gases from the combustion cylinder of an internal combustion engine between the piston and cylinder wall into the crankcase.

Leaking of gas pressure past the piston sealing rings.

Passage of unburned fuel and combustion gases past the piston rings of internal combustion engines, resulting in fuel dilution and contamination of the crankcase oil.

Combustion gases, composed of water vapor and unburned fuel, that leak past the piston rings into the crankcase during normal engine operation. These gases are removed by the PCV system to prevent the buildup of harmful acids in the crankcase.

Gases that blow past the piston rings during a piston's power stroke. These products form acid and sludge in the crankcase and cause smoke.