Abstract: A mechanical seal assembly for use in a coolant circulation system of a heavy diesel engine. The seal assembly includes relatively rotatable seal rings disposed in surrounding relationship to the pump drive shaft. One seal ring is non-rotatable with respect to the drive shaft and the other is non-rotatable with respect to the surrounding housing. These seal rings have opposed annular seal faces maintained in relatively rotatable contacting relationship due to the urging of a spring which acts against the non-rotating seal ring, the latter in turn being axially movably supported on a lightweight compact mounting member which stationarily seats within the stuffing box.