There was a recent thread about single piece fuel caps, and how to get the old gaskets
out of them. A number of suggestions came up. Pull them out with needle nose pliers,
dig at it with a screwdriver, cut it to pieces with a knife, burn it out (removing the
paint, too) or Goo Git or some kind of chemical.
Lots of ways to do it, I'm sure. I've done a bunch of them the past few months, and
the way I'm going to show works well, is fast, and pretty easy.

I use a bent end scribe. Similar tools are sold as O-ring tools at auto parts stores.
These are about $5, and a handy tool.

Put the pick end of the tool in the cap using one of the air vent slots to guide it.
This will help you push it in straight, and also keeps from putting pressure directly
on the thread peaks as you use the tool.

Put your thumb on the smooth part of the scribe where it's bent, and push down firmly
to drive the point of the tool in between the rubber ring and the side of the cap.

Keep pushing down, and lever the scribe handle away from the cap, and the point goes
under the gasket and peels up an edge, like so.
You might should wear leather gloves when you do this so you don't perforate your
tender body.

Now push the shaft of the tool through the gasket enough so you can use the edge of the
cap as a fulcrum, then pull out the gasket.

That's about it for getting one out. This takes about a minute.

Here, you can see why these older caps leak air. Just looking at the gasket while it's
in the cap, they often look just fine. However, they get weather checked similar to
old tires, and air comes out of the fount.

To put the new one in the cap, drop it in and make sure it is sitting down on top of
the flared inside part of the cap center.

With your thumbnail, or a popscicle stick, push in at one point. Then chase the edge of
the gasket around the center of the cap until it's in flat.

And that's it. Few minutes.
Dean