Parallel twin

A parallel twin (also inline or vertical twin) has two cylinders standing side by side across the frame, sharing one crankshaft. It is compact, simple and fairly cheap to build, so it powers many middleweight road, adventure and entry-level bikes, as well as classic British machines.

The engine's character depends on the crank angle. A 360° crank fires both pistons evenly; a 180° crank gives higher-revving smoothness; the 270° crank, used by many modern twins, gives an uneven firing order and a sound and feel similar to a V-twin, with good traction.

Balance shafts are often fitted to reduce vibration. Maintenance is straightforward: two sets of valves, two spark plugs and usually easy access to everything.