A type of piston engine employing four cylinders in a row, sharing a common crankshaft. This design is popular because it has perfect primary balance. The inner pistons balance the motion of the outer pistons, for little or no vibration.

Most cars and large-displacement motorcycles use this layout. It is also known as a straight four. The first motorcycle to use an inline four was the Benelli of the 1930s. Unfortunately, the complexity of the design somewhat defeated it and fours vanished into obscurity until 1968 when Honda produced the CB750. Fours still remained largely unappreciated until 1972, when Kawasaki released the seminal Z1.

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