This is on a Perkins/Sabre M92 4.236 Liter diesel engine in my CSY-44 Island Time. Here are images of the Heat Exchanger that I needed to repair or replace. After a week of trying to track one down, used or new, in the U.S., online and via dozens of phone calls, here were my options:

Heat Exchanger Mounted on the Engine

Cracked end cap and corroded end of the housing.
          I was able to find a replacement end cap and gaskets and have them shipped to me, and the tube stack was still good.

Body or Housing - Outer Side

Body or Housing - Engine Side

Corroded end of aluminum housing eaten away by electrolysis. This end was built back up using JB Weld poured into a form of masking tape. Crude, but effective. When you're working on a boat, you use what you have.

Rear End showing Bevel needed for O-Ring

End rebuilt with JB Weld Epoxy and hand filed back to correct dimensions and flat.
          I was still reluctant to create the bevel myself, so I took it back to one of the welders that I had asked to weld it back up. He saw me coming and started waving his hands and saying, "No, no, no, I told you I won't work on that. It's too far gone. I pulled the housing out of my backpack to show him that I didn't need the welding done, only some machine work. He looked at it and asked who had done the welding and I said, "I did...with JB Weld epoxy." He took it and said, "You'll never get it flat without machining it." He then pulled out a machinists square and stuck it on the end. He got a quizzical look on his face, then checked it from about 5 more points and said, "How did you do that?" I told him, "With a file." He said, "That's good, very good." and his whole attitude changed. He said, "You must have put in a huge amount of time." I said, "Yes, about 6 days," and he said, "I believe it, that's amazing. I've never seen anything done by hand that well." That made me feel pretty good! He took me a couple of doors down to another car shop to see if they had any idea how to get the bevel cut on the 3" opening, but that fellow had no ideas either.
                  On the way back to his shop he said, "You know, as well as you did the shaping of the flat part of the housing, which was very, very good, you could do the beveling yourself." I protested that I have no way to check the angle or get it smooth. He said, "I'm not kidding. You can do this. The hardest part is behind you. Doing the bevel won't take a third of the time. Just go SLOW. You don'd want to screw it up with all the work you already have into it." "You've got that right," I said, "That's why I wanted to find a machinist to do it." "No," he said again. "You can do this. Don't try to use any machinery like a drill or grinder. Do it with a half-round file or a sharp box knife and carve it. Go slow. You can do it."
I tried cutting it with a box cutter, and the first time around the hole went pretty well, but after that, I couldn't control the depth of the cut well enough, so I got out the files and used the half-round. Filing on the thin edge of the hole, the filing went very fast, but as I got further along, it slowed down, which is good. I really did want to sneak up on the depth, not cut too deep too fast. An first, I was just guessing at the angle of the bevel and could tell that I wasn't doing a very good job of that, so I switched to another idea I'd had. I got out a piece of stainless sheet that I used to scrape the wax off my skis with, thinking that if I stuck the corner of the right angle into the hole, that would give me the 45° angle I needed, and I could just rotate the stainless to scrape away the material to the depth I needed. I found a corner that was exactly 90°, sharpened the edges and tried it. It wouldn't cut. Next I burnished the edge of the scraper to put a tiny burr on the edges so it would cut better, and it did...for a turn or two. The JB Weld was hard enough to take the burr off. Back to the half-round file.

My guide for matching the 45° beveled angle.
            Now I had to figure out how to get that angle right with the file. I got out a short block of 2 X 4 and marked it with a pencil at 45° so I could lay it across the top of the housing, then match the angle with my file. I got lucky and realized that there was a ring down in the housing that the end of my file hit if I was cutting at the right angle, so I didn't have to keep the 2 X 4 in the way all the time.

End with the bevel completed

Tube Stack re-inserted into the housing.

O-Ring gasket in place after completion.
          The gasket will be compressed by the end cap against the side of the tube stack ring and the bevel in the housing to keep the seawater and the antifreeze coolant separated, and to keep both fluids from escaping; a three-way seal.