This is a Doble auxiliary engine designed for F-31 that was not used for such. It is double acting compound piston valve. HP is 1.531 inches, LP 2.575 inches and stroke is 2.00 inches. Engine has ball-bearing mains. Valve stroke is 25/32 inches maximum but there is a slip-feature eccentric for reversing and hooking up. The high pressure cylinder is jacketed to receive exhaust steam from the HP cylinder before it goes into the receiver to go to the low pressure cylinder. The piston valves are hollow with the center-cored passages allowing exit of exhaust steam which cools the valves. The very large diameter flywheel/ring gear is for driving a generator.