Cylinder bore diameter is the measurement across the inside of the bore. It can be listed in either inches or millimeters.
How is it measured?
The best way to measure bore diameter is with a Dial Bore Gauge.
If your block needs machined, the shop will hone the bore. When the machining is done, they will tell you the final measurement.
How does it affect performance?
The size of the cylinder bores will impact an engine's fuel efficiency and power output. Larger cylinders usually mean more power. They also require more fuel.
Bore diameter will determine what size pistons you need. It is also an important measurement used to calculate Engine Displacement.