Abstract: A pair of hydraulic stabilizing cylinders are supported on a common frame parallel to the main pneumatic drive cylinders driving counter-blow hammer impellers. The stabilizing piston within each stabilizing cylinder is connected to its adjacent impeller. Those ends of the respective stabilizing cylinders adjacent the impeller are connected to the opposite ends of the other stabilizing cylinders by fluid filled hydraulic lines. Because the cylinders and pistons are the same size, as the impellers move, fluid is transferred from one end of each cylinder to the other end of the other. However, should the impellers not move in synchronism toward their point of impact, that impeller tending to move ahead of the other will experience resistance from the fluid in the stabilizing system which tends to keep the impellers in step. In addition, means is provided to detect differences in position of the synchronizing pistons.