Valve-piston Crash
I believe this was a topic not too long ago, and so far as I know, it never was answered with certainty. I didn't try to turn over my 4.0 engine after I got it, and even at that, the chains were intact. However, the pistons in a 3.9 are featureless flat tops. The pistons in the 4.0 have very small valve notches just inside the circumference. While broken valve springs might let valves 'come down' far enough to touch, so far as I know, the closest a valve gets to a piston is when the exhaust valve(s) are chasing the piston down the cylinder.
When doing the first bit of research for my engine build, I visited a shop that specializes in rebuilding AJ engines. He had a pile of heads there and none that I saw had 'crashed' valves.
Don't take this as gospel. I haven't SEEN any. That's not quite to say that none exist.
Does anybody have first-hand experience of valve-piston contact?
KS