cleaning up the slant six head

I once left a tubular steel part in Muriactic way, way too long. The acid ate the weld seams in the tubing the most. Then it ate machined surfaces the next most. Based on this single event I have a shaky hypothesis that the acid attacks the metal with the highest internal stress the most. High internal stress = high metal removal; low internal stress = low metal removal.
Vinegar isn't as strong (or as dangerous) of an acid as Muriatic Acid is, so it's effects for equal soak time will be less.