A cylinder is the central part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work.

Cylinders may be sleeved (lined with a more rigid metal) or sleeveless (with a wear-resistant coating such as Nikasil). A sleeveless engine may also be called a “patent-bore engine.” A cylinder’s displacement, or swept volume, can be calculated by multiplying its cross-sectional area (the square of half the bore by pi ) and again by the distance the piston travels within the cylinder (the stroke).

The engine displacement can be calculated by multiplying the swept volume of one cylinder by the number of cylinders. Presented mathematically,

A piston is seated inside each cylinder by several metal piston rings fitted around its outside surface in machined grooves; typically, two for compressional sealing and one to seal the oil. The rings make near contact with the cylinder walls (sleeved or sleeveless), riding on a thin layer of lubricating oil; essential to keep the engine from seizing and necessitating a cylinder wall’s durable surface.

During the earliest stage of an engine’s life, its initial breaking-in or running-in period, slight irregularities in the metals are encouraged to gradually form congruent grooves by avoiding extreme operating conditions.

Later in its life, after mechanical wear has increased the spacing between the piston and the cylinder (with a consequent decrease in power output), the cylinders may be machined to a slightly larger diameter to receive new sleeves (where applicable) and piston rings, a process sometimes known as reboring.

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