The present invention relates to a power transmission unit for an automotive vehicle, and more particularly to a power transmission unit of the type which comprises a trans-axle casing rigidly secured to a cylinder block of an engine and having an upper compartment to contain a clutch for connection to a crankshaft of the engine and a lower compartment to contain a final drive gearing and a differential gear unit connected to each other, a transmission casing sealingly jointed at the upright front face thereof to the upright rear face of the trans-axle casing to contain a change-speed gear transmission drivingly connected to an output shaft of the clutch and coaxially connected to the final drive gearing, and an intermediate plate interposed between the casings to partly support thereon the output shaft of the clutch and the component shafts of the change-speed gear transmission.
In such a power transmission unit of the type described above, lubrication oil stored in the casings is stirred up by rotation of the change-speed gear transmission, the final drive gearing and the differential gear unit, and additionally the lubrication oil is transferred fore and aft when the vehicle is ascending and descending a hill. To provide an air breather on the power transmission unit, it is, therefore, required to prevent the flow of lubrication oil out from the casings and entrance of water in washing the vehicle.