Hold on. I have seen a tech do this before, and the pistons were not hitting the valves at all!
What happens is the chain, when not tensioned, can fall into the space between the crank gear and the oil pan. I always use a tensioner with a spring and no piston ratchet to prevent this.
I am sure this is what is up. You can wiggle and jiggle and possibly get the chain back up.
1991 GMC Syclone
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