My wife found a bolt head on the garage floor where I park. It's a 13mm stamped 8.6. I'm pretty sure it is one of the exhaust manifold bolts since everything underneath looks bigger or smaller. I don't see one obviously missing but I can't tear it apart right now. Can't see more till the heat shield cools. Can anyone confirm that's what they are? How serious is one bad bolt, from ignore to urgent?