Anyone know what could be wrong with my motor?

I have a VQ35de motor and 6spd trans from an '05 350z, but the motor will only do one full rotation. Meaning piston starts at the top and stops almost at the same point when it comes back up. I've taken the plenum off, valve covers, spark plugs, black oil pan on the bottom, and headers are off. It's out of the car and I have no harnesess. Nothing looks broken on the heads and no metal shavings, so I'm a little stumped on what it might be. If no one knows it's ok because I'll just probably take it apart and see what's causing it to not fully crank over(might not be until the summer time).