Greatpapabear,
You may not be doing the right maintenance to avoid having to replace the aftercoolers?? Read the following from an earlier post of mine, and follow up by reviewing the info on Tony Athen's site. He is an acknowledged Cummins (and other) marine diesel expert, who gives freely of his knowledge and experience.
"Freshwater flushing will save your engine's raw water cooling side,
reducing the need to "fully service" all of the components (aftercooler, fuel cooler, gear oil cooler, and heat exchanger).
However, even with freshwater flushing (which I do after each run when there is a decent flow of "dock water" available), the aftercooler will still require a complete "off engine" strip down every 3-4 years and every 2-3 if you don't flush. This is because even running ridlyme etc. through it will not deal with all of the issues. You have several dissimilar metals in contact with one another in saltwater inside the aftercooler (causing corrosion), o rings that could fail allowing salt water into the engine, and the air side gets "gunked up" by the oil particulates that are present in any engine room's air to say nothing about any crank case ventilation system.
Tony explains all of this in great detail on the above mentioned website.
If left without the proper regular servicing, the internal parts will corrode (basically weld) together (ruining the entire unit $$$), the air side will gunk up and start to slowly air starve the engine, and/or the saltwater side could clog up leading to overheating, to say nothing about the potential for saltwater to enter the engine.
After the complete off engine servicing, the aftercooler should be "pressure tested" to ensure no leaking.
To freshwater flush, just replace the sea strainer cap with one that has a garden hose (shutoff) adapter installed. Search for info on Tony's site for photos and how to's. With my setup it takes me about 10-15 minutes from start of getting the hose out to when everything is put away. I don't think just draining the aftercooler will result in much improvement when compared to flushing.
Hope you find this useful"
Unless your aftercoolers are totally different than mine (on a Cummins 6BTA), you need to disassemble them periodically or they can fail badly over time due to internal corrosion. Again, see Tony's site he has some photos of the worst he has come across, and it is not pretty