Where do they crack? Just so you know, the picture that Red showed is an exhaust manifold, they do crack because they are cast. They sometimes crack because of thermal cycling. A header is made from steel tube with a 3/8 steel flange welded on to attach to the head. A collector is welded to the four tubes, to make one tube, to attach exhaust pipe. The words "manifold" and "header" are two different things. If your headers did crack, which they don't usually, you can weld them. Not easy to weld a manifold.
