I don't know if this is only for diesel caps, I just did a quick google search and thought I would share
http://www.fueltanklock.com![](http://www.robond.com/uploads/robond_products.Photo/PUB%20FUEL%20TANK%20LOCK.jpg)
I bought a cheap locking fuel cap from Autozone back in 2004, not because I was worried about someone steeling my fuel, but because I was worried someone might tamper with my truck by pouring dirt or sugar or whatever into my tank
![Nervous :nerv:](https://board.marlincrawler.com/Smileys/marlin/nerv.gif)
Get a locking cap, you'll thank yourself
![Thumbs Up :thumbs:](https://board.marlincrawler.com/Smileys/marlin/thumbs.gif)