Abstract: Containers are filled with flowable substances while passing through a filling station. The plurality of nozzles and cylinders move through the filling station in unison with the containers. The cylinders have a free flowing piston movable under fluid action alternately to charge and discharge from alternate ends. A spool valve operates with respective cylinders under mechanical activation to charge and discharge cylinders and permit fluid from the supply to feed the respective nozzle. One cylinder end receives a stop rod which can be variably located into the cylinder and thereby to adjust the cylinder volume. The rod can be adjusted precisely as the vertical location of a mounting plate varies, which can be effected during operation of the filling machine. The cylinder length to diameter is substantially elongated to make for precision adjustments of cylinder volume as the rod is located variably into and from the cylinder. A highly precise adjustable filling system is achieved.