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Rubber Screw that fell out of toilet supply line

I needed to replace my toilet’s tank to toilet gasket, and in the process disconnected the toilet’s supply line. The rubber “screw like” object shown above fell out when I did this.

After finishing the gasket replacement, I forgot to put this back, and everything worked fine with no apparent leaks. But now I want to put it back, but I’m not sure what it does or what orientation it previously had. After researching and watching videos of flush valve and supply line replacements, it appears nobody else has a part like this in their toilets. I’m honestly beginning to think this object wasn’t supposed to be where it was.

Here’s an image of the disconnected supply line: Disconnected supply line

Here’s an image of the flush valve from inside the tank. Brand name: fluidmaster, probably installed ~2011. flush valve inside tank

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This appears to do a few things.

  • A backflow preventer. Note the nub on the "top", that should actually go down. Gravity and back pressure seats it on the inlet pipe to try to prevent tank water from backing up into the supply lines if there's a pressure loss or shutoff.
  • A flow limiter. Without it the toilet will flow faster, but will be more susceptible to water hammer. The automatic valve opens and closes quicker than humans, and flowing water has inertia. Changes in inertia cause force. That force can be destructive, which is what water hammer is.
  • A shock absorber, if it's made of rubber. This adds some compressibility to an incompressible medium, to help with water hammer effects, especially acting on that valve.
  • It will also work as a coarse filter, preventing dirt that might jam the valve open from entering. Ours has both a screw shape (as OPs) and a finer filter section, and came with two options for the screw thread pitch, for high and low pressure feeds. – Robin Bennett Oct 1 at 12:53
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Looks like a fill valve regulator.

Fill Valve Regulator

Pretty sure the metal washer is at the top when you put it in. Its actually an optional thing, and leaving it out might be just fine. It's weird that it fell out though.

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It is a flow limiter.

The cistern will have an overflow pipe in case the inlet valve fails. In an area with particularly high feed pressure the rate of filling in such an event might exceed the rate of drainage through the overflow. To prevent the resultant flooding the flow limiter can be fitted.

It's optional because if the feed pressure is poor you won't want to further inhibit the refill times.

All of the inserts I have encountered were simple plastic types. The device you've got is rather fancy, being in rubber and metal. I think the suggestions by @Harper justify that very well.

  • 2
    Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Sep 29 at 20:51
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    +1 for explaining why a toilet would want a flow limiter. I just presumed "faster fill = better" – Harper - Reinstate Monica Sep 30 at 5:51
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    Flow limiter can also reduce noise. The automatic valve often has poor geometry (optimized for short travel), so it makes a lot of noise if the flow is unchecked. – Agent_L Sep 30 at 17:09
  • The other reason for flow limitter may be the geometry of the automatic valve. It may be open when the level is low enough or when the level is too high (design flaw). If the flow is too high the level may easily overshoot the closing position and the tank is filled indefinitely. Lower flow rates anticipate this. In several cases I had to take care to "simulate" the flow limitter. – Crowley Sep 30 at 21:38

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