Dick; The magna-fluxing I am familar with worked by applying a magnetic field to opposite ends of the part. A crack causes opposite poles to form at the crack & the dye (magnetic) to concentrate there. This would of course not work on an internal flaw, which I assume is what you were referring to. The one I referred to in my bbl goes completely through the wall. It seems to me this is the type of flaw magna-flux detects.
I Didn't Say Everything I Said, Yogi Berra