I have never heard of using SHOUT to clean water jackets, just vinyl or cloth jackets
Seriously, that is new to me. For one thing, Shout is kinda expensive in the bottle form it comes in, so considering how many ounces one needs to do a system flush...well, why not use vinegar instead?
Yeah, that sounds ridiculous, but so does SHOUT. Considering what that stuff can do to your paint, I certainly wouldn't run it through those exposed alloy surfaces!!!