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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 an engine's characteristics. 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.

The cylinder and compression chamber are where compression and ignition occur in an automotive engine. When calculating displacement, one must take into account that a cylinder with limited dimensions can only compress and ignite x amount of fuel/air mixture, therefore putting out a limited amount of horsepower and torque. An engine can be taken to a machine shop and be custom machined to enlarge the bore. With a larger cylinder and compression chamber, as well as larger pistons, more fuel/air mixture can be ignited, producing more energy and power. Engine boring and porting are two of the most significant ways to boost a car's horsepower and torque output, although they are also the most time consuming and labor intensive.

Engine displacement is calculated by:

The term "bore" can also be applied to the bore of a locomotive cylinder.

Often, "ridges" can occur within the cylinder bore due to foreign matter entering the cylinders. Ridges should be removed using a professionally handled "bore brush". The "bore brush" is an electric tool that spins on a rod, which on the outside, small fine marble balls are attached. Once activated the bore brush should be pushed up and down in the cylinder bore as to remove any 'ridges' or foreign matter.

[edit] See also

• Stroke (engine)
• Compression ratio
• Cylinder head porting
• Reciprocating engine
• Two-stroke cycle

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