Just to state the obvious: A system with no expansion tank perforce must operate at higher pressure and temperature than one with. (Sort of, I know...) On most engines, this is controlled by the pre-set pressure-relief valve in the HX or radiator cap, which is a more vital and delicate piece of gear than it might first appear. The one on my boat seems to last about two years, but maybe that’s a sign that it’s time to acid-flush the block again? IIRC, the Atomic 4 wants a 5 PSI two-way cap, but it normally runs at only 160°F. Each engine should have a pressure specification for the cap. If it’s not a two-way cap, an expansion tank won’t work. None of our small tractor diesels have expansion tanks, I think. (Maybe the David Browning? I’d have to go look.) The radiator caps are one-way safety-release only. The Detroit Diesel I used to own did have one. The Beemer has no expansion tank and it is a royal PITA to purge that complex system and keep it happy. (High pressure and many, many possible failure points.) The previous owner didn’t realize that the pressure cap had failed and ruined the rest of the cooling system by the time that I rescued the car. Turned a five-minute twenty dollar fix into a months-long hundreds of dollars system replacement.