Abstract: A double acting working piston drives a connecting rod and crankshaft. A smaller valve piston is fastened to each end of the working piston. Each valve piston withdraws briefly from its bore so as to introduce scavenge air into a respective annular working chamber. The valve pistons have a centering means and are provided with novel radially restrained piston rings so as to smoothly re-enter their bores without impacting. Other novel features are: inlet nozzle and vanes, rear slide tubes for conducting coolant (such as water) to the pistons, an oil seal which permits the wrist pin to be full floating, and long tie bolts passing through water passages formed in the exhaust port bridges.