I have this under sink water filter: 3M Aqua-Pure Under Sink Full Flow Water Filter System Cyst-FF

It has push-to-fit 3/8 tubing going in and out.

It comes with sufficient hardware to go from the water supply to the filter (3/8 compression nut/ferrule/insert).

It comes with this "3/8 X 1/2 Faucet Adapter" to go from the tubing to the faucet:

3/8 X 1/2 Faucet Adapter 1

3/8 X 1/2 Faucet Adapter 2

However, my faucet has a fixed hose, and the other end is inaccessible to me (within my skill level / desire to take things apart):

photo of my hose connected to water supply:

3/8 tubing/hose connection from faucet to water supply

photo from the internet which I'm pretty sure is what I have:

3/8 compression hose female connector

What are my options for connecting the filter out to my faucet?

I'm pretty sure they are

  1. forego the provided adapter and get a push-to-fit-to-3/8 male compression adapter: https://www.johnguest.com/us/en/od-tube-fittings/specialty/brass/male-connector

push-to-fit-to-3/8 male compression adapter

  1. Use the provided adapter and get a male-to-male 1/2-to-3/8 adapter. This is the one that I've been researching for a long time and still not sure what this should be. OD, ID, MIP, tapered, compression, etc.


  • You have put a lot of work into this and have done some good research. However I would consider hiring a plumber as this will also need to be accessible to change the filter cartridge when needed. Also you apparently do not have the needed tools etc to do it properly.
    – Gil
    May 22, 2022 at 0:57

1 Answer 1


You've run into "progress" :)

For some decades US bathroom and kitchen faucets usually had brass metal male threaded inputs for their cold and hot water supply. These were usually 1/2", but sometimes 3/8".

These were then connected to the water supply shutoff valve (normally 3/8" male "compression", but sometimes 1/2") with flexible braid-reinforced supply hoses. Those flexible hoses have female connectors on both ends with rubber gaskets inside, making for easier leak-free connections and replacement if necessary. They are available in lengths from less than 12" to more than 6 feet, and with 3/8" or 1/2" or one of each connector on the ends.

In the never-ending quest for cost reduction and installation simplification, faucet manufacturers now make many of their faucets with braided flexible supply hoses permanently attached. This makes connecting each water supply a simple matter of securing one rubber-gasketed nut to the water shut off valve's standard compression-style male outlet (the ferrule and compression nut are not needed). This is what you have on your setup.

However, the water purifier kit seems to be made for the previous common situation, replacing a flexible supply hose in the middle, and comes with a real 3/8" compression connection for the shut off valve end, and a fitting for the post-filter end that would screw directly onto a standard male 1/2" faucet cold water input.

Your proposed Solution 1 - the push-on 3/8" to 3/8" male threaded compression end - seems the best one to me. The more fittings/connections/adapters you add, the greater the chance for a leak. As long as the end you've identified is made by a reputable manufacturer with a good reputation on push to fit fittings, you shouldn't have any issues. Be sure to read and carefully follow the installation instructions for both push to fit fittings.

PS - Also note that the rubber-gasketed metal connectors should be screwed on only to the point of "finger tight", then perhaps 1/4 turn further with a wrench. Tightening past this point can force the rubber gasket out of position and actually create a leak.

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