The stud itself is breaking. It shears off past the treads and where it is necked down, as soon as it starts making the ball thats where they shear off. Basically half the ball is sheared.
That is interesting. Assuming your rocker arms are aluminum I am surprised they don't break first. In either event, I wonder if your push rods are jumping out of the cup enough to catch the lip and break it off. Rods too short or not adjusted correctly? Could be the rods are rubbing on the head someplace setting up a harmonic that makes them jump out.
