Undersquare

In a reciprocating piston engine, the stroke ratio, defined by either bore/stroke ratio or stroke/bore ratio, is a term to describe the ratio between cylinder bore diameter and piston stroke. This can be used for either an internal combustion engine, where the fuel is burned within the cylinders of the engine, or external combustion engine, such as a steam engine, where the combustion of the fuel takes place outside the working cylinders of the engine.

A fairly comprehensive yet understandable study of stroke/bore effects was published in Horseless Age, 1916.

In a piston engine, there are two different ways of describing the stroke ratio of its cylinders, namely: bore/stroke ratio, and stroke/bore ratio.

Bore/stroke is the more commonly used term, with usage in North America, Europe, United Kingdom, Asia, and Australia.

The diameter of the cylinder bore is divided by the length of the piston stroke to give the ratio.

Stroke/bore ratio is less common than bore/stroke ratio, but is a term mostly preferred in German text (Hubverhältnis in German). It is used in some other countries too, such as Finland, likely due to German influence.

The length of the piston stroke is divided by the diameter of the cylinder bore to give the ratio.

The following terms describe the naming conventions for the configurations of the various bore/stroke ratio:

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