Firstly you’ll all have to excuse my ignorance but am I right in guessing that this “Y” type engine has the added complication of a one-piece head and cylinder block?
I suppose the two most important components are the crankcase and the barrels so in extremis everything else is sacrificial.
While we know that the pistons are solid in their bores are there any studs adding to the problem?
Heating an assembly is a popular and fairly simple method for loosening parts and can be successful even if it was not in this case. There is a practical upper temperature limit, it would generally not be safe to raise the temperature by much more than 250°C above ambient. The biggest “BUT” with a heating cycle is that while everything expands the things that are expanding the most are the pistons and they are expanding at a rate just about twice as fast as the iron barrels. Now while heating may well loosen things up the differential expansion is really working against us by crushing things together rather than opening things up. At a temperature of 270° (20°+250°) the piston wants to be 0.4mm (0.016”) bigger than the bore and anything ‘crusty’— corrosion, carbon, what-have-you, will tend to get crushed into the softer aluminium. While this might have the desired loosening effect it will also tend to turn the piston into an effective lap and thus more or less unfit for further use.
So how about going the other way and making things shrink? One thing that has not been suggested so far is Liquid Nitrogen. It may sound horrifically Frankensteinian but given sensible precautions it would be worth a try, it might give you frostbite but at least it’ll not poison you, dissolve you or for that matter the engine too! Liquid Nitrogen boils at -196°C, if the whole thing can be cooled that far the piston(s) will shrink by about 0.15mm (0.006”) more than the cast iron. Now that should loosen things up a bit! It would of course be wise not to just dunk the thing but to take the temperature down slowly to avoid stressing the thing too much, things get brittle as they get colder and cracks would just about put the tin lid on matters not to mention the safety aspects! To save on Nitrogen a liberal quantity of ‘cardice’ to cool the thing first would save a lot of gas.
While the engine is cold there might be an advantage to introduce into the barrels a mixture of a volatile and a very light oil which, with a bit of luck, will creep by capilary action down the sides of the pistons.
Time to befriend the local University or someone else with a SEM – EDX scanning electron microscope!
On the subject of the other materials suggested:
Aluminium and its alloys are prey to all alkalis; Caustic Soda will quite vigorously attack aluminium but will not harm iron or steel.
Aluminium will not be seriously affected by Phosphoric Acid but it does convert rust to an Iron Phosphate. The down side is that the conversion tends to expand the volume, quite the opposite to what is wanted!
Acetic Acid will have little effect on either metal.
The “active” ingredient in most cola is Acetic Acid, good for cleaning brass but not much else.