Tank Fit

Does not fit

Where the “universal” 400A is the wrong part

The Fluidmaster 400A is a good valve and the right answer for a large majority of tanks. “Universal” on the card means it covers the common case, not every case. Here is the short list of toilets where buying it wastes an evening.

1. Flapperless tower toilets — many Glacier Bay and Niagara models

Several Glacier Bay (Home Depot's house brand) and Niagara models use a flapperless design: instead of a flapper sealing a hole in the tank floor, a pivoting bucket or tower dumps its contents on flush. These tanks pair a specific fill valve with that mechanism, and the trip and refill geometry is not the 400A's geometry. Fitting a 400A here typically ends in a valve that fills to the wrong level, interferes with the tower's travel, or simply cannot be mounted where the old one sat.

What to do: find the model number stamped in the tank or printed on the paper label, and search that number plus "fill valve" on the manufacturer's parts page. Flapperless replacement parts are model-specific and usually inexpensive. Do not buy a flapper for these at all — there isn't one.

2. Short one-piece tanks

One-piece toilets — Kohler San Souci and Santa Rosa, TOTO Ultramax, many low-profile modern designs — often have a floor-to-overflow height below the 400A's roughly 9-inch minimum. The valve physically will not collapse short enough, and forcing it means the critical-level mark ends up below the overflow tube, which defeats the anti-siphon protection.

What to do: measure floor-to-overflow-top. Under about 9 inches, buy the OEM valve for the model. Kohler and TOTO both publish parts diagrams by model number.

3. Side-entry (rear or side inlet) tanks

The 400A is a bottom-entry valve: its threaded shank drops through a hole in the tank floor. Older tanks and a handful of current designs bring the supply in through the side or back wall of the tank. There is no adapter that makes this work properly.

What to do: buy a valve explicitly sold as side-inlet, or the OEM part. If you are staring at the tank and cannot tell, follow the supply line with a flashlight — the nut will be on the side, not underneath.

4. Pressure-assist toilets

Pressure-assist models (commonly built around a Sloan Flushmate vessel, found in commercial-leaning residential toilets) do not hold open water in the tank at all. There is a sealed pressure vessel inside. A gravity fill valve has nothing to do here.

What to do: if you lift the lid and see a black plastic vessel instead of standing water, stop and identify the cartridge for that unit. Also check for recall notices on older pressure vessels before servicing one.

5. Anything where the tank has a proprietary flush tower fill path

Some manufacturers route refill water through their own tube geometry integrated into the flush valve. The 400A will fill the tank, but the bowl refill can end up wrong, which leaves the bowl water level low and lets sewer gas smell through over days. It is a subtle failure that gets blamed on the wax ring.

What to do: check that the refill tube clips into the overflow tube at the same place the old one did, and confirm the bowl refills to its normal line after each flush.

The one-sentence test

If you lift the lid and see standing water, a bottom-entry supply nut, a rubber flapper on a 2- or 3-inch opening, and at least 9 inches from the tank floor to the top of the overflow tube — a 400A fits. Miss any one of those and look up your model number instead.

Where to find your model number

  • Stamped in the china on the inside back wall of the tank, above the waterline.
  • On the underside of the tank lid.
  • Behind the seat on the bowl rim on some models.
  • On the original paper label glued inside the tank — usually gone, but worth a look with a flashlight.

The four-digit Kohler "K-" number, the TOTO "CST"/"MS" number, and the Glacier Bay model number all lead directly to a manufacturer parts diagram. That diagram is the real authority on fitment, not a product page's compatibility list.

Next, in the tank