Are they not 16mm?
"Step 5- starting with the first headbolt, you absolutely want a 16mm 6point impact socket. (We dusted 2 craftsman sockets to find this out. they both broke in the same place, the same way.) The factory says the headbolt torque is 165 ft/lbs, and that very well may be, but something occurs under pressure. maybe the headbolts stretch, I don’t know, but the torque required to break these bolts free is WAY above 165 ft/lbs. I would even suggest keeping an extra 16mm 6 point impact socket on hand for fear of breaking. Always be careful of injector wiring and solenoids. These are very delicate and can be VERY costly if you damage one. Be sure the breaker bar has proper extension to clear the tops and wires to the injectors.
Do not use a ratchet wrench for this. A breaker bar is much more reliable. As well, we used a big cheater bar, because it was ABSOLUTELY necessary, on every bolt. Once you have broken the first head bolt, it will require about (3) 90 degree turns, the force will diminish each turn, but most likely require the cheater bar. You will then feel the headbolt release, and it becomes finger loose. There is one bolt at the back that you cannot get out; the second from the rear on the bottom row. To remove this bolt, remove nuts from motor mount, (4 nuts), then remove the 2 transmission mount nuts, then jack up about an inch, maybe a bit more on the bell housing. These will clearance the motor to get the headbolt out. Leave jack in place until stud is installed and nut is torqued through complete sequence. You can remove the jack after all studs on the passenger side are torqued to specified pressure."