Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

What's the metal disk and foam thing that's on the underside of a Tecumseh gas cap designed to do?

Discovered on my first gas refill that the parts on mine fell into the gas tank.  Fished them out but don't see how they stay on the shaft -- perhaps a clip broke off I haven't retreived.

Will replace cap, but for now was just wondering what that stuff does......