After weeks of research I've been unable to figure out what the thread size of a Pfister hidden aerator is. I want to remove the aerator and screw on an "instant-off" aerator device.

The Pfister aerator thread size is wildly non-standard. The outside diameter of a removed aerator measures as .73", or 18.62mm.
enter image description here

So naturally, I figured the threads were 3/4" 27F, and I needed a 3/4" 27M adaptor to screw into it:

Danco 3/4"-27M adaptor

But that didn't work. It's close, but it doesn't thread.

It seems as if the internal "hidden" threads inside the Pfister faucet MIGHT be MALE threads, with the threads on the removable aerator actually FEMALE 3/4" 27F. If that's the case, it's virtually impossible to screw an adaptor in, because it doesn't seem that any adaptor would be made with female threads on the outside.

Anyone ever run into this? Or have an idea what the actual thread size is supposed to be?

UPDATE: According to Price-Pfister, the threading of the inside of the faucet is 3/4"-18F, so an aerator or aerator adaptor would need to have MALE threads of 3/4"-18M. The problem is, 18M seems coarser than 27M, and if anything, the threads seem finer. In any case, I don't think there's such a thing as an 18M-to-anything-adaptor?

The model number of the faucet is: Pfister Weller LG49-WRO

View of Faucet from below

  • Do you have a model number for the faucet? Commented Sep 8, 2019 at 23:50
  • Would you give us pictures of the unscrewed areator, and of the faucet w/o areator? Commented Sep 9, 2019 at 12:41
  • @JimmyFix-it - it is "Pfister Weller LG49-WRO". But see original question for more detailed information. (Pfister says the internal threads are 3/4"-18F -- seems impossible) Commented Sep 9, 2019 at 17:28
  • @DanielGriscom -- see revised original question, with pictures, and assertion from Pfister that internal threads are 3/4"-18F Commented Sep 9, 2019 at 17:29
  • Yowza! Great documentation: let's see if you get a good answer. Commented Sep 9, 2019 at 18:29

3 Answers 3


3/4"-18 is UNS thread (Unified Special) which is a non-typical thread size. I am in the same situation trying to adapt a Pfister bathroom faucet to accept a swivel aerator. Still searching.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Commented Apr 21, 2020 at 13:16
  • I finally gave up, and bought a completely different brand of fixture for that bathroom! Commented Apr 21, 2020 at 16:29

I am sure that a 3/4-27 is not the correct size thread for a Pfister faucet aerator. It is some close variation to it. I was told by a master plumber that Pfister uses their own special threading and no one makes adapters with their threads.


Based on your information 3/4-18 is “English” or sae pipe is measured by inside diameter the number of threads is 18 threads per inch this is a “fine” pitched adapter as normal threads are 14tpi . It is not metric so you need a 3/4-18 adapter some will just say 3/4 fine.

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