Yanmar 2GM20F will not spin

[williamdieter]

Hey there folks, my 2GM20F was rebuilt in 2021 by a retired Yanmar tech. I Installed in the boat and used for 2 seasons without any issues. Started right up, ran great every time. We hauled the boat in December, and she's been on the hard in Virginia until yesterday. I winterized the motor before she was hauled, but forgot to redo it after we moved it over to the travel lift. I noticed the sea strainer was broken, so I assume the raw water froze.

Went to start the engine yesterday and it clicked, but didn't do anything. Battery was fully charged, I tapped the starter, same thing. Opened the compression levers and tried to turn the crank pulley and it didn't move. Turned it the opposite way and it loosened up a bit, and then I was able to rotate forward correctly for almost one full rotation but it felt like it hit something. Turned it backwards almost one full rotation and it stops in the same way.

I assumed that the heat exchanger might have cracked and leaked water into the cylinders. I removed the injectors and tried to hand crank the engine, but it stops in the exact same locations forward and backward.

Any idea what might be causing the issue, and what I should check next?