My turn to ask one instead of answer.. LOL.. Anyway, when we bought our 2003 A8 a few years ago, the gas cap would not seal all the way because the torque limiter (the thing that makes it slip to keep from over tightening) would slip at about halfway closed. I bought an el-cheapo Stant cap and it's doing the same thing. I had to put a small screw into the to lock it up so you can close the cap and get good seal. But, it gets hard to remove sometimes. I can see NO obstructions in the threads or in the filler neck that could be causing a drag on the cap. I'm thinking it could be a thread pitch mis-match with the caps (even the one that was on it before). Anybody have any technical information on this?

I'm going to order another cap, a Bilstein made part, so hopefully this will fix the issue. I guess we'll know more when it comes in.

 

My question is a valid one where I live in a bad neighborhood and have multiple vehicles (8) and moving is not an option. Therefore only bonafide answers please.
Update 2: Manufacturer doesn't make locking gas caps for the reason mentioned previuosly-have tried Stant locking gas caps and same result.