Definition
The internal diameter of a cylinder, such as the cylinder of a reciprocating engine. Bore is one of the two key measurements (along with stroke) used to calculate engine displacement.
Plain English
The width of the hollow tube inside an engine cylinder where the piston moves up and down.
Context Anchor
Seen in aircraft engine specifications, overhaul manuals, and discussions of cylinder size, wear, and engine displacement.
Derivation
From Old English 'borian,' meaning to pierce or make a hole. In machining, 'to bore' means to drill out a round opening — so 'bore' as a noun is the hole itself, or its diameter.
Why Pilots Care
Bore and stroke together determine an engine's displacement, which sets its power potential. A mechanic checking cylinder wear measures the bore to see if it has worn beyond service limits.
Intuition Check
Bore does not mean “boring” or “dull” here. It also does not mean the act of drilling; in this engine context, it means the measured inside width of the cylinder.
Example Sentence 1
The engine's bore measured 5.125 inches, well within service limits.
Example Sentence 2
Engine displacement is calculated using the bore and stroke dimensions.