its standard thred. to bust it loose alot of people including myself use a large ratchet or a breaker bar positioned on the bolt and wedged in the frame and the bumping the starter in neutral of course. impact wrenches usually work but this way has worked for me when they dont. carefull how you place the bar/ratchet so you dont sling it somewhere it shouldnt go like a rad or garage window.
I just had to remove the balancer bolt on a motor that had already been removed from the truck. My impact wouldn't do it. I removed the valve cover, placed a large screwdriver in the timing gear with a piece of wood wedged under it to protect the top of the head, placed a 6 point 19mm socket on the bolt, used a 3' breaker bar with a 2' cheater bar and viola! Well not quite viola....it took a few tries to get it just right but it came out.
Just thought I would update in case some one else runs into the same thing.