Good question.
Looking up the general recomendations on class 12.9 I see 12 to 13 ft lbs as the range published at internet sites.
So 144 in lbs.
Lets try thinking about other possibilities.
Do you still have the broken head?
Lets see a couple close ups.
Maybe it damage before you tightened it down, like a fracture.
Or maybe there was interference and the head wasn't contacting its mating surface.
Classic situation is junk at the bottom of a blind hole or a screw that is too long.
Its why we chase all the female threads
before bolting an engine or transmission together