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Bore and Stroke

Car culture term

What is Bore and Stroke?

Bore is a cylinder's internal diameter and stroke is the distance the piston travels inside it; together they set displacement and shape whether an engine leans toward high revs or strong low-end torque.

Bore is the internal diameter of a cylinder, and stroke is the distance the piston moves between the top and bottom of its travel, set by the throw of the crankshaft. Together, bore and stroke determine the swept volume of a single cylinder, and multiplying that by the number of cylinders gives an engine's total displacement. Two engines can share identical displacement while having very different bore and stroke dimensions, because the same volume can be achieved with a wide, shallow cylinder or a narrow, deep one.

When bore diameter is larger than stroke length, the engine is described as oversquare; when stroke is longer than bore, it is undersquare; and when the two are roughly equal, it is described as square. Oversquare engines generally allow larger valves and better high-RPM airflow, and their pistons travel a shorter distance per revolution, so they tend to rev higher and favor top-end power. Undersquare engines have longer piston travel per revolution, which typically produces more low-RPM torque and leverage on the crankshaft, at the cost of a lower practical redline.

These geometric choices are a core part of an engine's personality and are set at the design stage, since bore is limited by cylinder wall spacing and stroke is limited by crankshaft and block dimensions. A high-revving sport-bike-derived engine or a Formula-style race motor is almost always oversquare, chasing RPM and top-end horsepower, while a heavy-duty diesel or classic torquey V8 is often closer to square or undersquare, prioritizing grunt over outright rev range.

Related terms

HorsepowerTorqueRedlineCamshaftCompression ratio

Frequently asked questions

What is the difference between an oversquare and undersquare engine?
An oversquare engine has a bore diameter larger than its stroke length, which favors higher revs and top-end power. An undersquare engine has a stroke longer than its bore, which favors low-RPM torque but usually limits how high the engine can safely rev.
Why do high-revving sports engines tend to be oversquare?
A shorter stroke means the piston travels less distance per revolution, reducing peak piston speed and mechanical stress at a given RPM, which lets the engine safely spin faster. A larger bore also leaves room for bigger valves, improving high-RPM airflow.
Can two engines have the same displacement but different bore and stroke?
Yes. Displacement only measures total swept volume, so a large-bore, short-stroke engine and a small-bore, long-stroke engine can produce an identical displacement figure while behaving very differently, one revving higher and the other pulling harder at low RPM.
What is a square engine?
A square engine has bore and stroke measurements that are equal or very close to equal. It is often treated as a compromise design, balancing reasonable high-RPM breathing with reasonable low-end torque rather than optimizing hard for either extreme.
Does bore and stroke affect fuel efficiency?
Indirectly. Undersquare engines with strong low-RPM torque can be driven at lower RPM for a given speed, which often helps efficiency, while oversquare engines typically need higher RPM to make similar power, though other factors like compression ratio and combustion chamber design usually matter more.

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