Hi
I just did a rough drawing of the tool I was talking about. It had a hole for the rocker stud to poke through. To take strees away from the rocker stud I used 5/8th bolt with a M8 thread tapped into the head of the bolt, the bolt was fitted hex end against the rocker post and 5/8th nut was then used draw the tool down.
It worked in car as well if you put some compressed air into the cylinder.
Steve