3/8" socket. REVERSE THREAD. you or A previous owners may have tightened that thing up like the dickens trying to remove it. It may be super stuck. If so spray some penetrating oil all around the edge of the cap. put a pipe around the ratchet handel, and force it while someone holds the rachet firmly in the flywheel cap whole. If that does not work, heat it up with a torch, and try again.