Bore and stroke ratio

    If you are an automobile enthusiast you must be knowing about bore and stroke. In this post we will try to explore some lesser known facts about these two important parameters.
    Bore and stroke ratio, engine cc, stroke length, oversquare and undersquare engine


    The diameter of engine cylinder is called bore dia. The piston diameter is roughly equal to the bore diameter. In fact piston dia is little bit smaller than the bore dia. The clearance between the cylinder and the piston is filled by the piston rings.


    The length of piston travel from its top most position to its bottom most position is called stroke. Technically the length of piston travel from the top dead center to the bottom dead center is called its stroke.

    Calculation of engine capacity

    The capacity of engine can be calculated using the bore and stroke dimensions. For this we use a simple formula for calculation of  volume of cylinder i.e. 
             capacity of engine
              B=bore diameter
              L=stroke length
    If we take all the dimensions in cm, we will get cc of engine.

    Types of engine based on bore and stroke ratio.

    The bore and stroke are very important factors in designing of engines. Depending upon bore and stroke ratio there are three types of engines.
    1. Under square engines (L>B)
    2. Square engines (L=B)
    3. Over square engines (L<B)

    Under square engines

    The engines for which stroke length is greater than the bore length are called under square engines. In under square engines the piston has to travel a larger distance, so these engines have lower rpm range.  They have larger connecting rod and larger crank throw so they produce higher torque.  Since they have lower bore diameter, the distance between their center and outer boundary is comparatively smaller. That's why they are easy to cool and air cooling is generally sufficient for them. Greater friction is a major disadvantage with these engines.  Their pistons have to move larger distance in cylinder, so they experience greater friction. Due to smaller bore dia they have smaller cylinder heads, so  they can  not have larger or multi valves . Due to smaller valves they experience poor breathing.

    Over square engines

    Over square engines have larger bore dia. as compared to the stroke length. In these engines piston has to travel less distance, that's why they can achieve very high rpm. Due to less piston travel, they experience lesser friction per stroke. These engines response quickly to throttle i.e. they have crisp throttle response. Due to larger bore dia. they have larger cylinder heads. Larger cylinder heads can accumulate larger valves or more number of valves. They are either fitted with larger valves or 4 valves. Both of these arrangements improves breathing of engine. It means engine can intake more amount of air and can also exhaust more amount of burnt gases. This ultimately improves volumetric efficiency of engine. One major disadvantage of these engines is that, due to large bore dia. they are difficult to cool. This is because if bore dia is large the heat of combustion chamber can not be easily transfered to the cylinder block. So these engines must have some additional cooling arrangement i.e. liquid cooling.

    This was all about bore and stroke ratio. I hope you liked it. Please give your valuable feedbacks in comment section


    1. Regarding your synopsis about under-square engines having smaller cylinder heads and not having larger or multi-valves...perhaps you've never seen a Ford Modular engine?

      Also, regarding the last comment about over-square engines and a disadvantage of a large bore diameter being difficult to cool and, as such, requiring liquid cooling..... ever seen a Pratt & Whitney R-1340? Bore is 5.8 inches. Air cooled 9-cylinders. FWIW, cylinder surface area increases exponentially are bore size increases.

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