I AM WORKING ON AN 1987 470 170HP. WATER IN OIL . REMOVED WHAT NEEDED TO REMOVE TO TAKE OFF HEAD . ALL BOLTS CAME OUT WITH 1/2" IMPACT EXCEPT 1 THE ONE ON OUTSIDE CLOSEST TO THERMOSTAT I HAVE TRIED BREAKER BAR EVEN WITH 2' PIPE OVER IT -- HEATED BLOCK JUST A LITTLE WITH MAP GAS (DID NOT WANT...