The rings can also cause leaks when the manifolds are warped and they are assembled misaligned. That can be masked by the gasket and the close proximity of the manifold, especially in inexperienced hands. Then a dose of #2 Permatex is added in an attempt to stop the vacuum leak. That trick, when done neatly makes the misalignment that much harder to spot and the vacuum leak that much harder to find. Afterwards the alignment rings are typically unusable. If you find a heat-riser valve stuck in the "heat" position check carefully for warpage.
