the hex bolt is the tensioner. the tensioner screws into the side of the block or head. you cannot remove the tensioner out from under the valve cover.
removing the valve cover allows you to properly release the compressed tensioner. you must install the tensioner in the compressed position or you will damage the chain or guide. if you do not release the tensioner from its compressed position, oil pressure will eventually release it, but until you build up oil pressure, you have slack in the chain. some of the shortbussers have gotten away with it, but i wouldn't chance it on my own car.
the whole process took me less than an hour.
Q: What's the difference between a bolt and a screw?
A: First, you screw. Then, you bolt.