A flush valve is the part inside the tank of the toilet that moves the water into the bowl. Toilet flush valves come in different sizes from 2” to 4” depending on the manufacturing of the toilet. If you are thinking of buying a new toilet pay attention to which type of flush valve the toilet has because in most cases larger toilet flush valves are faster at moving the water resulting in a better flush. When changing the flush valve in an existing toilet make sure you get a flush valve that will work for that particular toilet because you usually have to stick to the size that is in there already. If you happen to find that the flush valve is an uncommon size, such as 3.5”, try contacting the toilet manufacturer for a replacement valve. They may end up sending you a new flush valve for free.