What is meant by bore and stroke?

Question

All engine specifications reflect a bore and stroke number.

Example

Aprilia Tuono V4 R

2012

Four stroke, longitudinal 65° V four , DOHC, 4 valves per cylinder

998.9 cc / 60.96 cu in

Bore x Stroke 78 x 52.3 mm Compression Ratio 13.0:1

There are characteristics of an engine that are reflected in the bore x stroke values.

Questions

What exactly is bore x stroke?

How do bore x stroke values effect efficiency in an engine?

Why is it so important for engine manufacturers to reflect bore x stroke numbers in their promotion of a vehicle?

Is there something about bore x stroke values that are important to consumers?

Answer 1

Bore is the diameter of each cylinder and stroke is the length that it travels when moving from bottom position to the top position. Thus if the engine has 1 cylinder with bore x stroke of 78 x 52.3 mm it's total displacement will be:

where π = 3.1416... and the bore and stroke must be in cm, thus divide them by 10 to get the right dimension. For your example it will be:

Engines with the bore bigger than stroke (like your example) are called oversquare engines, with the bore smaller than stroke are called undersquare engines and exactly equal are called square engines.
Oversquare engines allow bigger or multiple valves in the head and have less loss to friction due to the shorter stroke, but are only efficient at higher revs thus are more typical in high performance engines, like in your example for a sport bike. Bigger engines also have a larger bore even in trucks to limit the length of the stroke, thus limiting losses of the pistons rubbing the rings.
Undersquare engines allow for a better torque in low revolutions where the longer stroke will not affect performance so much but the fuel burn will be more efficient due to the lower volume when the air/fuel mixture is compressed, leading also to lower fuel consumption. Diesel engines also are massively undersquare engines (typically around 3:1 ratio) since those engines have a very high compression ratio compared to gas engines (around 22:1 ratio).
A long time ago the tax on cars in England was applied proportional to the bore size, thus even high performance cars like Jaguars had unusual long strokes to allow a high displacement while still paying lower taxes.

IMHO many other factors will dictate the performance and fuel consumption of the engine, thus only looking at the bore x stroke ratio is meaningless, yet by rule of thumb you can say that big bore gives you higher performance at high revs and long stroke gives you lower fuel consumption at everyday usage.

Answer 2

Stroke is length of stroke and bore is diameter. Yes it matters in linear speed of cylinder. Big block engines will go with really big bore as you get to a limit on stroke.

So important? It is just a facts about the engine.