While the starter is strong enough to run right through interfering valves, turning it by hand won't be. I would remove all the rocker arms so the valves will be closed and see if you can turn the motor. The rockers won't be removable on valves that are being depressed so you might have to back the motor up to get all of them.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician