Pretty sure it's regular thread (I tried both anyway....), it's the idler not the tensioner, I haven't gotten around to it yet (midterms) but i'm going to just murder the pulley and leave the bolt be and run with no sc for until I can get a nut welded on then i'll warm up the engine (maybe that will help) and hope a breaker bar gets it out. I really don't want to snap it and have to drill it in that tight spot...

part number on that pulley? and maybe the bolt? (Preferably one with a hex...)