When I replaced the heads on my car I tried to set the rockers to have zero lash. I interpreted this from the chiltons manual as meaning that the pushrods should be snug and you ca't move or twist the pushrods anymore.

Now I here that if you overtighten them that there can be problems. Is that true and how do you know that you did?

 

The valves will lift off the seats and stay open.

 

They would stay open? Without the rocker pushing the spring down would they remain closed?

 

That's the idea.

 

Hydraulic lifters use zero lash but also have a preload inside the lifter. You adjust the rocker until the pushrod becomes snug without overtightening to push down on the pushrod. Then tighten the rocker down an additional 1/2 turn to set the preload. After setting the preload, the lifter will bleed down and the pushrod won't be tight any more. That's normal and just how hydraulic lifter work. Don't tighten it any more.

With a solid cam, you always have valve lash. If tightened to zero lash, the valve will never close onto the seat.