[fanaudical]

I'm not a chemist, I just play one at work occasionally. (I'm a mechanical engineer with some history with chemical handling for various industries.)

I agree with your friend that HCl is a reasonable choice for what you're doing (providing you're careful with using low concentrations). I was suggesting phosphoric because concentration control gets less critical. Your process described above seems sound for neutralizing the acid after cleaning (which is important). Hopefully your soap is of the caustic variety to help kill the acid.

Trust me on this one - Pumping HCl (even dilute) through those lines at high flow rates may result in tapering (wall thickness reduction) at the entrance/exit of the line and potentially in tight bends. Rough guess based on line size is that ~250 mL/min is a safe flow rate. I recommend orienting lines during flush so that you flow from bottom-to-top to purge bubbles.

Not that I think this is a concern here, but one interesting thing that happens with HCl cleaning of high-strength steels is hydrogen embrittlement. I don't recommend cleaning any fasteners with HCl unless you're also going to do some things to mitigate that.

My understanding is that radiator shop "hot tanks" used sodium hydroxide (aka "caustic soda") back in the day before aluminum radiators became so prevalent. Sodium hydroxide cleaning solutions work well on copper, iron, steel, but etch(or worse) aluminum allows.

Those lines are beautiful, by the way.