The best way really is just to take the parts to a machine shop and have them hot tanked.
Never heard of this,we used to hot tank the alluminum heads for the race car before we checked for cracks and re-planed them all the time.
I tried the water method, apparently not enough because here is the result after opening the head :
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I also tried to clean with a copper brush + solvents, ... without success.
Before you take it apart, with it running, useing a spray/mist bottle. Spray water into the throttlebody while you keep it running with the throttle. Do this with the motor warmed up for 4 to 5 minutes, it will loosen up a lot of what's in there and blow it out the exhaust. Do a oil change when your done with the repairs. Good luck