Thanks Dave, i have the engine dissasembled infront of me, no visible damage or foreign particles, the crankshaft is not visually bent or something like that, but something curious:

It seems like the piston sleeve is "tighter" on the top, cause when the piston goes all the way up, it pushes the sleeve and it pops out about 1 or 2mm, when the piston goes down it pulls the sleeve back in.

What do you think? Do i "break it in"... AGAIN?