I have a head that's off the engine and stripped down to do a little mild port clean up, resurfacing, etc. And the coolant passages (little internal compartments within the head) have a lot of scale, rust, corrosion build up in them. In the past this always seemed to clean out well when heads returned from the machine shop, after being worked on and cleaned up. But when I asked the machine shop about it, I was told the EPA has banned them from using agents to the point their cleaners cannot remove any of the scale, etc. Basically I was told there is nothing that can be done and to just ignore it. That sounded odd to me so I went to two other shops, but was given the same answer. There must be a method of cleaning out the coolant passages? Perhaps something the shops cannot use, but we can? With the head stripped down it can be soaked, but in what? Perhaps the rust remover products like "Evap-O-Rust", or a acid solution, or "CLR" (calcium, lime, rust remover), or ??? In past experiences most of those things don't seem to do a lot with other stuff but I'm open to ideas. One factor is being able to clean out the coolant passage areas but not harm the combustion chambers, valve guides, seats, mating surfaces, etc. There are too many little odd-shaped openings everywhere to try and contain a solution only within the coolant passages, rather than submerge the entire head. What has anyone tried, and how well did it work?