one of the heads we are going to use on our rebuild has a crack at the very top of the valve guide, you can kind of see it in the pics (pics not too good, sorry) it is cracked in the narrow portion of the vavle guide, not the thicker portion where it meets the head (if that makes any sense) is this useable, repairable, ????, from what I understand these are desireable castings so I would like to use them, thanks...