I suggest anyone spending the time to build turbo headers or logs for any siamese port head, add a few extra bolts between the center ports. This solved a major annoyance for me. I had my flanges built out of 3/8 stainless, added additional material around all the ports in the drawings, and added the bolt holes. If the flanges were being made, I would strongly suggest doing this. Even if the end user doesn't want to thread the heads, there wouldn't be any interference, or issues not using the holes.