Pop up gas cap is a direct replacement for the stock Harley gas cap. Of course this custom pop-up gas cap can fit bobbers and most custom motorcycles using the same type of fuel tanks. When installed, these caps are perfectly flush with the tank surface. To access the filler bung just push the spring-loaded pop-up center and turn 90 degrees to release, then unscrew normally (note: what looks like a paint saver ring is actually the base where the spring loaded cap recess, it is one piece with the cap and unscrews with it). Custom fuel tanks with single filler bung will need only our right side pop up gas cap. Motorcycle fuel tanks with dual filler bung will obviously need a left and a right side pop up gas cap. Stock gas tanks with fuel gauge unit will need our adapter and two right side pop-up gas caps (one for the right side and one to go inside our adapter to replace the OEM fuel gauge unit). This custom pop up gas cap is made out of stainless steel and mirror polished. When we creted this product our purpose was to clean up as much as possible the look of custom motorcycles; the same concept that was considered when we created our Universal Motorcycle Kickstand with Internal Spring, Turn Signal Grips and 3D Rex Motorcycle Mirrors with Internal Adjustment.