I RECENTLY REPLACED THE IMPELLER IN THE LOWER END AND WHEN PUTTING IT BACK TO GETHER, I FILLED IT WITH OIL, AND NOTICED OIL LEAKING OUT THE SMALL HOLD AT THE BOTTOM OF THE LOWER UNIT BEHIND THE PROP. WHAT IS THIS SMALL HOLE, IT IS ALWAYS OPENED? ANYHOW I FILLED IT WITH OIL, AND IT QUIT LEAKING...