the rod will shake back and forth a lil bit when i jiggle it. i can get the crank the budge. it's locked up somehow and somewhere in that engine. the piston has black stuff on top of it. i didnt find any metal shavings on top of the crank. but i cant do anything to get it move. if it's the bearings can i just replace them? or do i have to replace the entire crank assembly?