Outer Limits 4x4 Board

hey guys got a 5l diesel running fine till i done an oil change went out for a play blow the turbo have had it rebuilt since the new turbo went on motor has had bad blow by out the oil cap have removed the head c
gasket looked to be ok bores look new very little play in the pistons an cant find anything wrong with valves or any cracks in the head its driveing me up the wall any ideas cheers