the rocker cover should be held on by a nut and stud configuration if you have a bolt in place it could be there has been an issue before. what ever you have in place try removing it and then inspect the state of the thread in the hole which should house the stud it may be that the thread can be re tapped to achieve a reasonable anchor point. if this is not possible then the second option is to enlarge the hole re tap with a larger size and enlarge the hole in the rocker cover. third option is screw in a stud into the old threaded hole after applying a smidgen of liquid metal to the stud and leave to harden the rocker cover is not tightened up excessively (or should not be) so should be fine. this is not something i have found on any heads before as it is not a highly stressed point so should not fail normally.
