There are two local machine shops that can work my cast cylinder
heads. Both will mill them if warped for 30 bucks a piece. But to
crack check, one uses magnaflux at 3.50 per cylinder. The other will
"water tank" them for 30 bucks per head. (I think her called it
presssure testing.) He said on the water tank they submerse them and
look for bubbles and it will tell if there is a crack inside the head
as well.

Is there a big difference? Magnaflux is cheaper I guess.