Bore-Stroke Ratio and engine shapes

What Is Bore-Stroke Ratio and Square Engine Design?


 | 3 minutes read

What is Bore-Stroke Ratio?

Bore-Stroke Ratio is the ratio between the dimensions of the engine cylinder bore diameter to its piston stroke-length. The cylinder bore diameter divided by the stroke-length gives the Bore-to-Stroke ratio. The ‘Bore-stroke ratio' is an important factor which determines engine’s power & torque characteristics.

Bore-Stroke ratio = Cylinder Bore diameter / Piston Stroke-Length



The engineers also classify and categorize the automotive engines according to their shapes. It’s the shape of cylinders as seen from the side of the engine.

Principally, there are three types of geometrical shapes around which the engineers design conventional engines with. They are –



  1. Square Engine
  2. Under Square Engine
  3. Over Square Engine

What is a ‘Square Engine’ and how Bore-Stroke Ratio affects its design?

An engine is called a ‘Square Engine’ when its cylinder bore diameter & stroke-length are almost equal which forms a geometrical figure of a perfect ‘square’. The bore-stroke ratio is almost 1:1 in Square engine design.

Square Engine Design
Square Engine Design

For e.g. an engine with a bore diameter of 83mm and stroke-length of 83mm which forms a perfect square. It provides a perfect balance between speed and pulling ability.

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