Tank Fit

TOTO

G-Max and Power Gravity: the flapper is not generic

TOTO's gravity flush systems — G-Max on Drake and Ultramax, Power Gravity on commercial-leaning models — use a wide 3-inch valve with a TOTO-shaped flapper. The size looks familiar in the store aisle. The sealing profile and the hinge are not.

Identify what you have

  • G-Max: a wide 3-inch flush valve with a large soft flapper, big trapway. Found on Drake (CST743 family), Ultramax, and many two-piece TOTO toilets from the late 1990s onward.
  • Power Gravity: TOTO's higher-flow gravity system on models aimed at heavier use. Similar service logic, different part numbers.
  • Double Cyclone / Tornado bowls: a bowl rinse design, not a tank mechanism. It tells you nothing about the flapper; go by the flush valve in the tank.
  • Washlet models: the seat is electronic and irrelevant to the tank. See seats if you are planning a bidet seat.

Why "3-inch" is not enough information

A generic 3-inch flapper is designed to seal on a generic 3-inch seat. TOTO's seat profile and the flapper's float/hinge behavior are tuned so the flap stays open for exactly as long as the flush needs, then drops. Fit a generic one and the usual results are: a slow weep that shows on the dye test, or a flush that ends early and leaves the bowl needing a second pull. Neither is dramatic enough to be obvious, which is why people replace the same flapper three times.

TOTO THU-series flapper (by model)OEM G-Max / Power Gravity flapper and sealtypically $12–$25
Fits
The specific TOTO model on your tank stamp. TOTO publishes parts diagrams keyed to the CST/MS number — for example the Drake CST743 family — and the flapper is listed there as a THU part number. Buy that number.
Does not fit
Cross-shopping by 'fits TOTO' text alone. TOTO revised flush valves across production years, and Power Gravity parts are not the same as G-Max parts even where both are 3-inch.
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Korky 3-inch for TOTO (54BTP class)Aftermarket TOTO-compatible 3-inch flappertypically $9–$14
Fits
Korky lists specific 3-inch flappers as TOTO-compatible replacements and they generally perform well on G-Max valves in good condition. A reasonable pick if OEM is backordered.
Does not fit
Tanks where the seat is already scaled or scored — an aftermarket flapper is less forgiving of a rough seat than the OEM part. Also confirm on Korky's own fitment lookup with your CST number before buying.
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Service notes specific to TOTO tanks

  1. Water level. TOTO marks a fill line on the overflow tube or inside the tank. G-Max flush quality falls off noticeably below it. Set the fill valve to that mark rather than eyeballing an inch of clearance.
  2. Chain slack. TOTO chains are short. Half a link of slack still applies, and the excess should be clipped, not coiled.
  3. The fill valve is usually replaceable with a universal. Most two-piece TOTO tanks take a standard bottom-entry valve — measure floor-to-overflow and use the chooser. One-piece Ultramax tanks are the ones to measure carefully.
  4. Do not use in-tank bleach tablets. TOTO's own care guidance warns against in-tank cleaners; they attack the rubber you just paid for.

Find the CST or MS number first

It is stamped in the china inside the tank, above the waterline, and often printed on a label on the tank's inner wall. With that number, TOTO's parts documentation gives you an exact flapper and seal listing. Without it, you are guessing — and on this brand guessing costs flush performance, not just a leak.

What TOTO owners usually actually need

In rough order of frequency: a hardened flapper after five to eight years, a fill valve at the end of its life in hard water, and a loose tank-to-bowl connection that weeps at the bolts. The flush valve body itself rarely fails. If a new correct flapper does not hold the dye test, check the seat surface — see the seat section on the flapper size page.

Next, in the tank