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What is Meant By Bore and Stroke ? Explanation About Bore-Stroke Ratio?


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:


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.The geometrical shapes of engine is divided into 3 types which are discussed below :-




  1. Over Square Engines.
  2. Under Square Engines.
  3. Square Engines .
Over Square Engines :-
Engines with the bore bigger than stroke (like your example) are called Over Square Engines.Manufacturers also refer it as a ‘short-stroke’ engine. Over square 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.Such engines will run faster and there for not that reliable in long term. Eg: Race car engines, Bikes.
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.
Advantages of ‘Over-Square’ Design:
  1. Less frictional losses & load on bearings.
  2. Higher engine speeds.
  3. Reduces engine height thereby lowering the bonnet line.

Under Square Engines :-
Engine with the bore smaller than stroke are called Under Square Engines. Under Square 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 Under Square engines (typically around 3:1 ratio) since those engines have a very high compression ratio compared to gas engines (around 22:1 ratio).
Eg:- Engine in Trucks & Locomotives.

Square Engines :-
In Square Engines 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.

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

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.

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