I'm late to the party on this one - I don't look in as often as i did.
However, I do have to clean and decarbonise engine parts frequently...
What I used to use was Tergo - a product of the Auckland gas works. An evil smelling byproduct of the gas producing process, it would strip everything. Carbon, paint, skin...Now discontinued for environmental reasons.
The engine reconditioners have had to change to soak/cleaning solutions which are safe. As have I.
There are three commercial cleaning supplies companies locally. All have an automotive parts soaker/cleaner - usually around $30 for 5 litres of concentrate.
The one I favoured has again gone off the local market and it's replacement isn't quite as good.
Used at a 5 : 1 ratio with hot water, a set of CBR600 pistons - 1988, heavily carboned, oil stained....soaked for 3 days, came out exceptionally clean and ready for crack testing. This stuff doesn't touch paint or skin and used at the common 10 : 1 ratio cleans cases and finned barrels very well.
So what i'm saying is - touch base with the professional cleaner suppliers before you use caustic or any of the more dangerous cleaning methods....