because I had to open up the holes to gasket match my D port brodix heads.Dawg,
If they are "new" and they are cracked ....... why would you not call the manufacturer and have them replaced under warranty ???????????????
the stuff your thinking of is thermosteelJB Weld may not be rated high enough for a header repair right at the head. There is another product at the autoparts store right next to the JB Weld that is rated for 1300 degrees but the name fails me. It's in a small round plastic container/tub and packaged on a piece of cardboard like most tubes of goo. I've used it to repair a hole in the water pan on my smoker that sits right over the hot coals but never on a car.
Unfortunately, you're probably going to have to ugly them up a bit to prep the surface. You may be able to use a chainsaw sharpener bit in a Dremel to rough it up right along the crack so as not to completely wreck it.