5.7 manifold errosion

[cdavisdb]

Just pulled the manifolds and risers off my 5.7. Engine was installed in 1998, almost unused till I bought it in 2009. I replaced the risers then, but the manifolds looked almost new, so I did not remove them.

4 seasons later is now. Manifolds still look ok to me, but the seat on the block is erroded around the bottom of the exhaust for the center two cylinders on both sides, see pics. The erosion is about half way across the sealing surface. I don't remember seeing this on previous engines.

Is this normal? If not, do I need to to more that just bolting a new set of manifolds on? I could clean it up real good and marinetex it and sand smooth, but that's a lot of work. Ideas?