i dont see how that is, unless the crack is deeper then it looks, cause in the photo it appears to end right at the valve margin. but from what i can see, you should beable to machine all of the crack away for the new seats. if the valves are large enough, which it looks like they are, the 2 seats should overlap eachother.... which effectively removes the crack. but if other machinest say it cant be done.,... then your screwed.