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A flat-eight engine, also called a horizontally-opposed eight, is an eight-cylinder piston engine with two banks of four inline cylinders, one on each side of a central crankshaft, 180° apart.
In a flat-eight engine, the connecting rods for corresponding pistons from the left and right banks may share a crankshaft journal. A boxer-eight engine is a special case of a flat-eight where each piston's connecting rod has its own journal, and each pair of opposed pistons moves inwards or outwards at the same time.
Flat-eight engines have been used in automotive, aircraft, and marine applications.