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Piston/ diaphragm / Navel ?? If you give the type of flushometer then a plumber can tell you exactly how it is put together
Flushometer or tank and bowl?
By discharging a predetermined rate of flow and GPM using either a piston assembly or diaphragm
one inch for a water closet, 3/4 for urinal
Adjust the flushometer / Fill valve / flapper