Abstract: A pair of well service pumps are mounted on a trailer or skid or bob tail truck chassis in a back-to-back configuration. Each pump has a housing with a cylinder within which a plunger is reciprocally moved. A crankshaft mounts rotatably in the housing perpendicular to the cylinder. The connecting rod connects the crankshaft to a crosshead of the plunger. The axis of rotation of the crankshaft is offset from the cylinder axis. One of the pumps is inverted relative to the other pump so that its offset is the same but in the Opposite direction. The pumps are rotated in the same direction by prime movers.