These "Pop-Up" gas caps allow you to give your gas tank that real Custom Smooth Top look, as seen by the worlds leading Custom Bike Builders. They are easy to open or close, just push in the cap, twist and the cap will pop up and is ready to unscrew. No matter if you have a steel or aluminum gas tank, Zodiac has these Custom goodies with your choice of aluminum or steel weld-in bung. Caps are vented and available in aluminum with a top-quality show chrome finish, polished Stainless Steel, or natural aluminum to apply the finish of your choice. Bungs have a 2 5/16" (59mm) outer diameter, caps are 55mm in diameter.