[dtw]

That is the 'death bolt' (though not a bolt at all). It is one of the intermediate shaft bearing web fasteners. If it is leaking - be VERY careful tightening the nut. In mag-case engines, it loves to pull out of the case threads. It is very long, which means a thread repair is near impossible with the engine assembled (the threads anchor on the other case half from what you are seeing in that picture).

When the stud pulls, some advise just tapping the case and slapping a bolt in it until rebuild time.