Hi - I am repairing a 1982 OMC Sterndrive with 2.5L 4 CYL. I noticed water in the engine oil. I had the head magnafluxed and it came back good. I am going to replace the head gasket next. Where are the most common places for water leaks to occur. This is a open cooling system with fresh water. I have heard the manifold and riser can crack but how do you pressure test them?