Recently a thread appeared that discussed locking gas caps, or more specifically the lack of a solution. The only known cap (from 70's BMW 2002) fits and appears to lock, but doesn't actually lock to the car. I was able to come up with a solution that modifies the fuel filler ring to allow the locking tab on the BMW cap to have a place to lock against. My locking gas cap solution works for the Z's up to the 74 260.  The key is to look and see how many screws hold the filler ring