Definition
The leakage of combustion gases past the piston rings into the crankcase of a reciprocating engine. Blow-by occurs when the piston rings fail to seal against the cylinder wall, allowing high-pressure gases from the combustion chamber to escape downward during the power stroke.
Plain English
When an engine fires, hot gases push the piston down. Some of those gases sneak past the piston and end up in the lower part of the engine instead of staying above the piston where they belong. That leak is called blow-by.
Context Anchor
Seen in piston-engine maintenance, compression checks, oil condition discussions, and reports of crankcase pressure or breather discharge.
Derivation
Literally describes what happens: combustion gases blow by the piston rings instead of being held back by them.
Why Pilots Care
High blow-by signals worn piston rings, which reduces power, contaminates the oil, and can lead to engine damage if left unaddressed.
Grounding Statement
If the seal around the piston is weak, some of the pressure that should push the piston down escapes into the lower engine case instead.
Intuition Check
Blow-by does not mean air flowing past the outside of the airplane. Here it means combustion gas leaking past an internal engine seal.
Example Sentence 1
The mechanic suspected blow-by after the differential compression check showed air escaping into the crankcase breather.
Example Sentence 2
Excessive blow-by was pushing oil out the breather tube during flight.