Because this is an Interference engine, somehow you are 'out of time' between the cams and the and the pistons.
I had the a similar probem when I tried to install a new timing chain. Learning curve

It doesn't seem wise to turn the engine by hand, with the tensioner removed.
How do the gear teeth look? Pointy?

Additionally, It has been written here that the crank hub can shear a key, and really confuse the issue.

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1970 280S M130 engine- good runner
1971 250 M130 engine- #2 rod bearing, gone
1971 280SE (blown engine,parts car)
1977 German 280S W116-only 33 years old