It's not a big deal to pull the engine apart and see what the bearings feel like. But he said the crank spins freely and piston slides up and down smoothly. Who knows. I'd have the whole motor in pieces on my table, but that's just me. I like to work on stuff like that.
A had a similar problem. engine stated ran fine for a few seconds and stopped. it would stop at TDC every tine and was loose on the bottom. Turned out to be a bad bearing inside. The bearing cage shattered. into many pieces and that was the problem. No damage just replaced the bearing.