When we put the motor in on an automatic car the flexplate somehow bound up. It was nearly impossible to turn over. Unbolted the bell housing bolts and I heard it pop back. We paid closer attention on our next approach and all was well, car has been on the road for a while. This was an auto 220 gas W115. So not the same situation, but I have seen them bind up. However if the car was previously on the road running, it wouldn't randomly bind up that way. I reread you post on my computer and my comment on that front is probably irrelevant.

1 bar of oil pressure at idle is fine, it should peg the gauge on the highway though.

Are you sure you don't have a blown headgasket and you are temporarily hydrolocking the motor? You don't see many blown HGs on 615s, but I have seen motor hydrolock from tons of fluid in the cylinders from a bad HG.

Rebuild parts will probably prove tough to find. I have a 220D motor in Ohio out of a '72 220D 4speed that ran for a couple minutes until it ran out of fuel, its overall condition is fairly unknown but I could attempt to compression test it on the bench if you decided you needed a motor and you're near Ohio.

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