Pulled my manifold off couple days ago and after ripping off the alternator i found out that someone in the past had already removed that 15mm bolt and never put it back. How important is it? What's its thread so i can get a new one from the hardware store if need be?

 

That bolt really sucks to get to which is probably why it was left off. I have had no issues leaving it out on multiple builds. If you do want to put one back in, it should be 10mm X 1.5 thread pitch but I dont quite remember the length.