The 501 flapper is built to last in any water condition with its chlorine-resistant properties. Unlike flexible flappers, the 501 flapper features a solid frame that doesn't twist for the best seal every time, so you never have to worry about jiggling the toilet handle again! The 501 universal flapper is best suited for use with 3.5 gal per flush toilets made prior to 1994. Only Fluidmaster has toilet flappers with built-in Microban protection. Not just simply a coating, Microban is specially built into the construction of our flappers to fight flapper breakdown and combat bacteria.