I have a boat that is 1978.the part # na001545. Any vendors that rebuild these things? It is a heat exchanger.
Depending on the style, it may be best to simply disassemble the unit, inspect for cracks, and replace the tube unit with a factory remanufactured unit from the engine manufacturer. It's not difficult at all for Cat engines, anyhow.
Rebuilding a heat exchanger for a boat motor, could be as simple as unbloting the risers and end caps and cleaning it out. I know a guy in North Alabama where I'm from. He is a diesel mechanic. He may be willing to do this if it is sent to him. It would take me a while to round up his phone number. You, if your mechanicly inclined, could take care of your heat exchanger you self tho.
Take it to a radiator shop. They can check it out, test it and tell you if it can be rebuilt.
