Am working on a core box for an engine cylinder head. It is a one-off affair, so I am trying to do this the easiest way. Am going to use the actual head from another engine as the outer pattern (adding material for shrinkage) and am making up a core pattern for the passages. The foundry I go to uses no-bake core sand. Question is can I make the core box mold from RTV? I can house the RTV in a wood box to give it rigidity. If so, making the parting lines for the core box would be real easy with a sharp knife. Comments/suggestions?? Tried to search this on the internet and found zip.