I sold one of my kit gas caps to a guy in need and ordered a replacement cap with some other parts.
It went on fine, but it didn't want to come off.
After playing with it for awhile I got a bit physical with it, and that broke the little spring tensioned flange that locks the cap to the tank off.
The cap came off fine then, it went back on fine too, but obviously it no longer locked in place.
Well, after looking at it awhile and playing with the spring inside I figured I could just put something else in there, so that's what I did ;-}
The gas tank grove on mine is exactly 35mm wide.
The cap body itself is 29mm wide which means whatever you put through will only stick out 3mm on each side of the cap body.
With that little of a clearance something smooth would likely move so I rummaged around and found the perfect thing, an unused intake manifold stud.
It was about 5 mm too long so I used a grinder to get it to 35mm.
It worked like a champ.
The tricky part is the spring inside the cap body.
It is the same diameter as the inside of the cap body and it goes all the way up, this is what provides the tension on the cap to hold it down as you twist the cap on so you don't want to go between the coils, you need to be on top of the spring like this.
To get the stud in I used a standard screw driver with a blade big enough to easily hold the spring down on one side to get the stud through to the other side, than a 16 penny nail on the other side to hold that side of the spring down to get the stud through a bit.
Then I pulled both tools out and just used pliers to twist in the stud until it was equal on both sides.
It seems to work as well if not better than a stock flange, the stud threads should stop any lateral movement, it left just enough room to move for travel going down on the locking bevel on the tank, and it sure as heck won't break again.
So, if you find yourself in same situation don't toss the cap, fix it ;-}