Definition of Bore & Stroke

An engine's displacement is directly determined by its bore and stroke.

In an internal combustion engine, the piston is housed within a compartment known as the cylinder. The cylinder is where the air-fuel mixture is compressed and ignited, which delivers power to the engine. The terms "bore" and "stroke" describe mechanical measurements related to cylinders and pistons, respectively.

Bore

The bore of a cylinder describes the measurement, in millimeters or inches, of the inside diameter of the cylinder. Diameter is the width across a cylindrical object. It is an imaginary line drawn through the center of a circle that connects two opposing points on a circumference.

Stroke

The stroke of a piston describes the distance, in millimeters or inches, a piston travels from either the top of the cylinder to the bottom of the cylinder, or from the bottom of the cylinder back to the top. Either direction--up or down--traveled by the piston is considered to be a single stroke.

Undersquare or Oversquare Engine

An "undersquare," or long-stroke, engine is one whose bore measures shorter than its stroke. An "oversquare," or short-stroke, engine is one whose bore measures wider than its stroke. A "square" engine has a bore and stroke that are equal.

References

• NASA: Bore and Stroke
• BoostTown: Bore and Stroke Overview

Resources

• Find Articles: National Dragster - Big Bore Or Long Stroke: Which Is Better?
• Ducati Tech: Displacement Finder