Tank Fit

Tank Fit · 11pm edition

Your toilet is running. Let's name the part.

Four questions, then a part number and an honest note about what it does not fit. No brand war, no lab claims — just the geometry inside your tank.

Running toilet diagnostic

Question 1

1What is it doing?

Stand in the room with the door closed and listen for ten seconds.

Question 2

2Dye test: put a few drops of food coloring in the tank, wait 15 minutes, don't flush. Any color in the bowl?

This is the single most useful test in toilet repair. It separates a leaking flapper from everything else.

Question 3

3Does a trick stop it?

Only tells us about the flapper and chain.

Question 4

4How old are the guts, and is your water hard?

Chloramine and scale set the replacement interval more than brand does.

Answer all four and the result appears here — nothing is sent anywhere, this runs in your browser.

Why fitment, not brands

Almost every "best flapper" article compares Fluidmaster to Korky and stops there. That is the wrong axis. Both companies make good rubber. The reason a repair fails at 11pm is that the part in the bag was built for a 2-inch flush valve and your tank has a 3-inch one, or your Kohler uses a canister seal, or the "universal" fill valve cannot go short enough for a one-piece tank.

So every page here leads with the measurement, names the SKU when it is public, and says out loud what the part does not fit. If your model is one where only the OEM part behaves — Kohler Class Five is the clearest example — we say buy the OEM part.

The five-minute triage, in order

  1. Dye test. Color in the bowl = seal. No color = above the waterline.
  2. Waterline. It should sit about an inch below the top of the overflow tube, or at the molded mark. Water spilling into the overflow tube runs forever and sounds like a fill valve problem because it is one.
  3. Chain slack. Half a link. Too tight lifts the flapper, too loose gets pinched under it.
  4. Measure the drain opening before buying any flapper.
  5. Read the tank stamp. Under the lid or on the back inside wall — the date code tells you which production run of a long-lived model you own.

A running toilet is expensive, and that part is documented

The EPA's WaterSense program specifies 1.28 gallons per flush for labeled toilets, versus the 1.6 gpf federal maximum, and notes that leaks are a large share of household water waste. A flapper that passes water continuously can run through more water than the rest of the bathroom combined, which is why a $6 part is worth doing tonight rather than next month. Check the EPA WaterSense site for the current specification text.

Tools you actually need

  • A 6-inch adjustable wrench or channel locks for the supply nut.
  • A sponge and a small bucket — the tank never empties completely.
  • A flashlight, because tank stamps are always in shadow.
  • Food coloring. That is the whole diagnostic kit.

Next, in the tank