Cylinder Bore
The inside diameter of an engine cylinder, which is measured to calculate engine displacement and piston clearance.
The cylinder bore refers to the inside diameter of the cylindrical chamber in an engine block where the piston travels. It is a critical dimension in engine design, measured in millimeters or inches. Along with the piston stroke and number of cylinders, the cylinder bore is used to calculate the engine's total displacement. It must be machined to extremely tight circular tolerances to ensure a proper seal with the piston rings.
Over time, the friction of the moving piston and combustion forces wears down the cylinder bore, making it out-of-round or tapered (wider at the top where combustion pressure is highest). This wear allows combustion gases to blow past the piston rings into the crankcase (blow-by), causing loss of compression and oil consumption. When rebuilding an engine, the bores are honed to restore a cross-hatch pattern or bored out to a larger size to remove wear ridges.
Symptoms of a worn cylinder bore include blue exhaust smoke, high oil consumption, loss of engine power, and cylinder misfires. Mechanics diagnose cylinder bore condition by performing a cylinder leakdown test and using a dial bore gauge to measure the bore diameter at various heights. If a cylinder is worn beyond limits, the block must be bored to fit oversized pistons or fitted with cylinder sleeves.