A flat-four engine (also known as a horizontally opposed-four engine or boxer engine) is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed pistons moves inwards and outwards at the same time.

A boxer-four engine has perfect primary and secondary balance, however, the two cylinder heads means the design is more expensive to produce