First I have a full closed cooling system. This is a 7.4l mpi with 740 hours on it. I yanked my risers and they are not terroble themselves but going to replace them. Now my manifolds have surface rust pretty decent. In fact the screws into the block are rusted and will need to be cut off and drilled out. That's the extent of the surface rust. Now the inside of the manifolds and riser spacers looks absolutely fine. Barely any surface rust on the interior walls at all. Is the rust on exterior surface enough that I should replace them now? Or should I continue to use them as they are working fine?