ALL engines with an even number of cylinders have pairs of pistons moving in the same direction at the same time. They're called "companion cylinders". One is on compression while the other is on exhaust, on the upstroke, and they run intake and power strokes on the way down- - - -exactly 360 degrees apart in the firing order.
Inline sixes run smooth because the three cylinder pairs are exactly 1/3 turn apart- - - -naturally balanced. 4-cylinders, V8's and V6's are trying to shake themselves to pieces all the time.
Jerry
