Balance shafts are most common in inline four-cylinder engines which, due to the asymmetry of their design, have an inherent second order vibration (vibrating at twice the engine RPM) which cannot be eliminated no matter how well the internal components are balanced. Four-cylinder flat engines in the boxer configuration have their pistons horizontally opposed, so they are naturally balanced and do not incur the extra complexity, cost or power loss associated with balance shafts (though the slight offset of the pistons introduces a rocking couple). This vibration is generated because the movement of the connecting rods in an even-firing four-cylinder inline engine is not symmetrical throughout the crankshaft rotation; thus during a given period of crankshaft rotation, the descending and asc