Originally Posted by dubcac

There's no way it could have gone into the engine. Was it the bolt type or just a nut that goes onto a stud from the head? If it was the nut type, check to make sure that the stud isn't broken off inside. If it was the bolt type, check to see if part of the bolt is still inside the head. In either case, you would have to use a bolt extractor kit to remove what's left. Hopefully it just backed itself out. Any bolt/nut for a B-series header will work...LS/B16/GSR/ITR.