Just had this same problem with the 6mm hex nuts on my alternator mount. Take a socket that bareley fits over the 6mm bolt, (cant remember if I used 10 or 12mm.) Use a socket that has multiple small teeth inside of the socket instead of the ones that have a six sided cutout. Put an extension on the socket and then hammmer the socket onto the head of the 6mm bolt. Pound it on good. Once its on tight, use your ratched to unscrew the hex nut. Work like a charm.