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Bidet T-valve doesn't fit? Here's why, and the fix

A bidet T-valve that won't fit is almost always one of five things: the threads don't match, the fill valve's threads are worn, there's no room between the tank and the shut-off valve, a rigid supply pipe can't stretch to the new height, or the toilet is skirted and the inlet is hidden. Each has a cheap fix, and none of them needs you to force anything.

First, stop turning. Plastic threads cross easily, and a cross-threaded fill valve is the one mistake here that turns a ten-minute job into a new part. Then work out which of the five you have, with the checks below.

What the T-valve is supposed to do

The T-valve (or T-adapter) goes between the bottom of your toilet tank and the supply line from the wall. It has three openings:

• Top: a female thread that screws onto the tank's fill valve shank, the threaded plastic stub sticking out under the tank, left side.
• Bottom: a male thread that your supply line screws onto, exactly as it used to screw onto the tank.
• Side: a smaller outlet for the bidet hose, often with its own little shut-off.

In the US and Canada, the top and bottom are both 7/8 inch "ballcock" thread, the standard for toilet tank connections. The side is usually 3/8 inch or 1/2 inch to match the bidet hose. When a T "doesn't fit", one of these three joints, or the space around them, is the problem.

Which problem do you have?

Threads that don't match

How to check your fill valve thread

Turn off the water at the wall, flush to empty the tank, and unscrew the supply line from the bottom of the tank (towel underneath, there's always a little water). You'll see the fill valve shank: a threaded stub about an inch across.

• A US or Canadian toilet built in the last few decades: 7/8 inch, almost always. If the supply line you just took off says "7/8 ballcock" on its nut, that's your answer.
• Some toilets, often imported or older ones, use 1/2 inch, 15/16 inch, or other sizes. A plumbing parts reference lists 1/2 inch and 15/16 inch as the less common sizes T-valves come in.
• Outside the US, sizes are different again: UK, European and Australian toilets connect differently, and the T usually goes at the wall valve.

The quickest test: take the old supply line's nut and screw it back onto the shank by hand. It should spin on smoothly. Now try the T the same way. If the supply line nut spins on and the T won't, the T is the wrong thread or a bad part. If neither spins on smoothly, the shank threads are the problem.

Plastic shank, metal T

Some kits ship a brass T. A metal female thread on a plastic shank needs care: start it by hand, perfectly straight, and stop at hand-tight plus a quarter turn. People report all-metal T's that won't start on plastic inlets at all; when that happens, a plastic-bodied T in the same size usually goes on fine.

Worn or crossed fill valve threads

Look closely at the shank. Flattened, chewed or shiny threads on one side mean it's been cross-threaded before. Bio Bidet's troubleshooting guide says the same thing: if the shank threads are crossed or worn, the T won't sit snug, and the toilet's fill valve needs replacing. A new fill valve is a standard hardware store part with instructions on the box, and it's a normal DIY job. If you rent, it's a maintenance request.

How to start a thread without crossing it

1. Hold the T straight up, square to the tank.
2. Turn it backwards (counter-clockwise) slowly until you feel a tiny click: the threads just dropped into line.
3. Now turn it forward by hand. It should spin easily for several turns.
4. Any stiffness in the first two turns means it's crossed. Back it off and start again.

The supply line won't reach

The T adds roughly an inch of height between the tank and the supply line. A flexible braided hose bends to take that. A rigid pipe doesn't.

• Rigid chrome or copper pipe: don't try to bend it down. Replace it with a flexible braided toilet supply line from the hardware store. You need the right fittings: 7/8 inch ballcock on the toilet end, and whatever your wall valve has on the other (3/8 inch compression is common in the US). Measure the old pipe and buy a line a little longer, 12 or 16 inches usually covers it.
• Flexible hose that's just too short: same fix, a longer braided line.
• Old rubber or plastic hose: replace it anyway. You're already there with the water off, and it's the part most likely to fail later.

Never stretch a hose tight to make it reach. A hose under tension pulls on the joints, and that's how a drip starts.

No room between the tank and the shut-off valve

Some toilets have the shut-off valve right under the tank, with only an inch or two of supply line. The T and its side outlet need more room than that.

Three fixes, in order of how easy they are:

1. A low-profile or short T. Some T-adapters are more compact than the one in your kit. Measure the gap from the bottom of the shank to the top of the shut-off valve and compare it with the T's height on the product page.
2. A T at the wall valve instead of the tank. These connect to the shut-off valve's outlet (often 3/8 inch compression) and feed both the toilet's supply line and the bidet hose from there. Bidet sellers sell them for exactly this case.
3. Turn the side outlet. If the T fits but its side outlet points into the wall or the bowl, many T's let you angle the bidet hose connection, or you can use a short hose with a 90-degree elbow. Don't kink a hose to make it fit.

The bidet hose won't fit the side outlet

The side outlet and the bidet hose have to match. Common sizes are 3/8 inch and 1/2 inch, and kits from different brands don't always agree.

• Use the hose and T that came together in the same box. Mixing a hose from one brand with a T from another is the usual cause.
• Some kits include a second adapter for the hose in case the size is wrong. Check the bag of small parts.
• Bought a separate T? Buy the matching hose, or a size adapter, rather than forcing it.

Skirted or concealed toilets

A skirted toilet has smooth sides that hide the trapway, and often the water inlet too. Sometimes you can reach the fill valve by lying on the floor with a flashlight; sometimes it's boxed in.

• Inlet reachable but cramped: a skirted-toilet T-valve kit. These are made to fit where the fill valve is hard to reach.
• Inlet hidden: put the T at the wall shut-off valve instead and run a longer bidet hose to the attachment. Check the wall valve's size first; some use 1/2 inch, which needs a matching T.
• Measure the hose run: from the wall valve, around the toilet's side, up to the attachment's inlet. Buy a hose a little longer than that.

Our main fit guide covers the seat side of skirted and one-piece toilets.

The T is on, but crooked or rocking

• Short fill valve shank: on some toilets the shank sticks out only a little below the tank, so the T grabs only a turn or two of thread. That's not enough to hold it square or seal. A longer fill valve is the real fix.
• Two washers: the tank's old cone washer can stay stuck inside the old supply line nut, or on the shank. Plus the T's own washer, that's two, and the T sits proud and crooked. Use one, seated flat.
• No washer: also crooked, and it will leak. Check the T's top opening for a flat rubber washer.

Water side fine, but the attachment itself rocks or the lid won't close? That's the seat side: see bidet attachment doesn't fit. On a one-piece Kohler with a tight back, bidet to fit a Kohler toilet covers the model check.

What not to do

• Don't use a wrench on plastic threads. Hand-tight plus a quarter turn at most.
• Don't wrap thread tape on a joint that seals with a rubber washer. It can stop the washer seating.
• Don't cut, file or drill the fill valve shank to make a T fit.
• Don't bend a rigid supply pipe. Replace it.
• Don't leave the water on overnight with a T you're not sure about. Close the T's valve, or the wall valve.

Parts list, by problem

Take the old part, or a clear photo of the threads next to a tape measure, to the store. It saves a second trip.

When to call a plumber

• The wall shut-off won't close, or it leaks at the stem when you turn it.
• The fill valve needs replacing and you'd rather not.
• The supply comes out of the floor with no valve at all.
• You rent: shut-off valves and fill valves are the building's to fix.

Once the T is on, the rest of the job is in how to install a bidet attachment, and if it drips afterwards, bidet leaking from the T-valve finds the joint.

Where ASSJET fits

Our attachment uses the standard US 7/8 inch T, like most kits. When it goes on sale, we'll tell you before you pay if your toilet needs a different part. Until then, find out which god of the bathroom you are with the ASSJET quiz.

FAQ

Are all bidet T-valves the same size?

No. Most US kits use 7/8 inch at the tank, but 1/2 inch and 15/16 inch exist, and the side outlet for the hose can be 3/8 or 1/2 inch.

Why won't my bidet T-valve screw onto the toilet tank?

Usually it's not lined up straight, the threads don't match, or the fill valve's threads are worn. Start it by hand, turning backwards first until it clicks into line.

Can I install a bidet with a rigid supply line?

Yes, after you replace the rigid pipe with a flexible braided supply line. The T adds height the pipe can't stretch to.

What if there's no room for the T-valve under my tank?

Use a low-profile T, or a T that goes on the wall shut-off valve instead.

Do I need thread tape on a bidet T-valve?

Usually not. Joints with a rubber washer seal on the washer, and tape can stop it seating.

Not at the plumbing stage yet? Take the bathroom god quiz first. More answers in our bidet guides.

Sources

1. How to Install a Bidet T-Valve and Troubleshoot Problems, EngineerFix. https://engineerfix.com/how-to-install-a-bidet-t-valve-and-troubleshoot-problems/
2. Bidet T-Valve for Skirted Toilets, BidetKing. https://bidetking.com/t-valve-for-skirted.html
3. Common Bidet Problems: A Bio Bidet Troubleshooting Guide, Bio Bidet. https://biobidet.com/blogs/news/bio-bidet-troubleshooting-guide
4. How to Install a Bidet, Bob Vila. https://www.bobvila.com/articles/how-to-install-a-bidet