gary wrote:
Very good. My question was how long should the unpainted portion of
the piston rod be when the door is open.

So it was. I got lost in other details. The answer is, the piston rod
should be fully extended but no one can tell you in inches, you'd have to
measure it.

Your problem is that you tried to place the new door bracket where the old
one was. As you said, you had to "use a lot of strength" to pull out the
piston to mount it in the old location. It took strength because your new
door closer was shorter than the old one (they come in different lengths)
and/or the brackets were configured differently.

To fix things, remove the door bracket and reattach it with the piston fully
closed.