sometimes when pulling the starter, the engine just locks up. Somethign is up with the piston i think. What could it be and how can i fix it?

It is probably hydraulic locked; the cyclinder is full of liquid fuel which it can't compress. Simply loosen the glow plug by one full turn (no more than that) and then start the engine with the plug loose. As soon as it's running, retighten the glow plug. Job done.