I'm guessing the dog bone fell out as you put the piston in. Just a guess.

The other possibility is a broken band.

I know, it was all mentioned and you checked tension. But it sounds like it has to be opened again.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles