Inline Four

An inline four has four cylinders side by side in one row, usually mounted across the frame on motorcycles. Honda's CB750 of 1969 made the layout popular, and it became the classic Japanese sports and naked-bike engine.

Inline fours run smoothly and rev high, producing strong top-end power and a distinctive howl. The downsides are width, weight and complexity compared with twins, and many have less low-down torque. Some, such as Yamaha's crossplane designs, use an uneven firing order to change the feel.

Maintenance includes keeping the throttle bodies or carburettors synchronised and checking valve clearances at the specified intervals, which takes more work than on a single or twin.