Bore, when used in the context of piston engines, is a measurement of the diameter of the holes bored into the engine block for use as cylinders. The ratio of stroke to bore is a major factor in defining the characteristics of an engine. Generally, smaller bore yields fast-revving engines with low rotational mass and therefore higher rev limits with later torque peaks and overall higher top end power at the expense of torque throughout the operating range. Larger bore with longer stroke tends to make higher torque engines with lower rev-limits, with the increased rotational mass leading to slower reving.
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