To recap the story thus far, I got an 81 300SD with a transmission condition. The engine starts, but the flex plate is shredded. All 6 torque converter bolts sheared off. Main shaft has 1/4" of axial play and considerable radial play

Many thanks for all the diagnostic help.

What I noticed when comparing the old transmission with the replacement is that there are no dowels or guiding pins in the old transmission. They aren't on the block either. So the transmission was located on the block by the bolts in the bolt holes which probably allowed several mm of radial misalignment. If it were misaligned enough to cause flex plate fatigue, wouldn't there have been noticeable vibration?

The replacement transmission goes in over the next few days. I hope there are no more surprises.

Sixto
91 300SE
81 300SD