The Universal M18 diesel engine has two ports at the exhaust outlet on the block. The flange for the '84 Catalina 27 exhaust has a square hole centered over the top port, but covers up the lower port. What is the purpose of that port? I have the gasket ordered, so I'm not sure whether the replacement gasket has a hole there or not (a PO had made a gasket that did cover the flange material covering the lower port). I am planning on simply putting Permatex on whatever shape gasket comes to my door, but I'm curious what use that port may have in other applications of this engine. I'm also planning to use antiseizing on the pipe threads for the U-tube pieces, though I hope my work lasts as long as the original. Problems with this exhaust system seem to be common on older boats judging from the number of posts in the archive. My particular problem was that the U-tube had been reinstalled over on its side at some point in its life, possibly allowing water to flow back into the engine. I couldn't free the union to rotate the pipe, so I'm replacing everything. It's incredible how much corrosion can resemble welding! The nipple between the flange and the union had also taken on a rakish tilt over time. I suspect that had I not 'discovered' this problem, I would have had an exhaust filled bilge at some point this summer.