Originally Posted by long-gone

First, I'm inclined to go along with the suggestion that it was on the ground when you started. From the look of it, it hasn't been in a hole for a while. If it was, the portion of the threads that were in use would be much cleaner.

Having said that, it appears that it would take about a 4 or 5mm hex bit, either of which are used on the throttle linkage brackets, the throttle body and various brackets securing components to the manifold and head such as the EGR tube, etc. Maybe also look at your cruise control unit where it bolts to the block.