360 partially rounded head bolt help

one of the head bolts on my 360 is partally rounded. its too small for the 3/4 to get a good grip, too big for 11/16, and the 12 point 3/4 socket just spins on it. it almost seems like its at 18mm not but none of my metric sets have that size. can anyone else help me? all my other head bolts came out fine with the breaker bar