I took mine apart two years ago, probably the first time ever since the boat was new in 2002. I didn't have an engine manual then, so it was a case of take a look and if it looks ok then leave well alone. Apart from removing the crud, mussel shells and pieces of impeller vanes that had dropped off from the time before I owned the boat, it all got put back together without anything being renewed. Both ends of the heat exchanger were taken off, and I made a note that whilst there was some corrosion within the exhaust elbow, that it was not too bad in my opinion, though I'm not an engineer. I've looked for a stainless exhaust elbow, seen some advertised with KeyPart, and I'm looking to change that elbow in the spring when the engine overhaul is due. As for "O" rings, they seemed to fit well enough so they were not changed. Maybe I'll change them in the spring now that it's been mentioned, but I'll take a look first at the engine manual that I have now, to see if there is anything else it recommends before I get too carried away.