Hy johnhull, grade 8 bolts are very strong, but they are also brittle, when the manifold expands from heat it could actually break (pop the heads off) grade 8 bolts. I learned this lesson very early in my career, grade 8 bolts are not the best for shock loads or heated components, the "factory" bolts for the manifolds that I have seen have no head markings on them, which means they are not that strong, but can accept many heat/cool cycles, hope that helps.
