Boxer twin

A horizontally opposed engine configuration with the two pistons opposing each other, commonly found on BMW twin-cylinder motorcycles. The term comes from the resemblance of fists coming towards each other. Also known as a pancake engine.

A 1967 BMW R50/2 longitudinally mounted flat-twin engine, with tank removed. Note that the cylinders are not truly in line but displaced by the width of one crank pin and one crank-shaft web.