Yep headgasket is shot.as for the posture of the piston has nothing to do with this.it would cause compresion increase but it hasnt as its just been checked. .cant be cracked head because surely the head was pressure tested when it had skim and valve seals.iv had this confirmed by 3 garages ive just been on the fone to.amd for in writing i was told nothing about head gasket issues.all i was told was piston 4 was a tad higher but was told to get a multi layer gasket to prevent damage.nothing to do with head gasket blowing.