A gear case assembly for a marine propulsion system has a gear case having a first end adapted to connect the gear case to a remainder of the marine propulsion system, and a second end disposed opposite the first end. A driveshaft is disposed at least in part in the gear case. A propeller shaft, operatively connected to an end of the driveshaft, is disposed at an angle thereto. The end of the driveshaft and at least a portion of the propeller shaft are disposed in a transmission chamber defined in the gear case. An expansion port is defined in the gear case. A container, defining an expansion chamber, is disposed outside the gear case. The expansion port fluidly communicates the transmission chamber with the expansion chamber. An outboard engine is also disclosed.