Alright, I found it. It goes directly into the block, 3-4 inch from the manifold toward the left as you look into the bay from the top. It is partly hidden from view. 19mm, mine was very tight, I used a breaker bar. It probally not fully serviced before. The Prestone that was "changed" a year or so ago by the previous owner's mechanic did not look too great when removed. The inside of the radiator looked clean though. I flushed it twice with tap water then filled with G-05 and distilled. No sense getting too crazy.