Tried Emailing, but got no reply. Their list of engine head bolt/stud kits mentions nothing about the 4D55/56 diesel engines. My feeling is that there is a stud that they make that would fit, but finding it would involve cross referencing the application based on the type of stem instead of part number. Its very rare that anyone with access to the part numbers is willing to do that leg work for you. So for now, I'll probably have to reuse the old bolts.