I am trying to install a Culligan FM-25 filter onto a faucet, and it does not seem to fit. I am able to screw on the filter, but it is not secure, and water bubbles/sprays out the top. It appears that the faucet head is too long (or has too many threads)? I know that my filter can accommodate 55/64"-27 M faucet spouts. If the filter at least partly screws onto the faucet, then isn't the faucet 55/64"-27 M as well? I can't find any examples of thread spacings other than 27 threads per inch for 55/64, so I'm assuming the faucet size is also 55/64"-27 M (but I could be wrong).

I have a picture of the faucet head as well as one of the water filter and aerator that I've removed from the faucet. As you can see, the aerator appears to be much deeper than the filter mount. Not only is there an unthreaded region above the threads, but I can count 4 visible levels of threading above the washer, whereas only 3 levels are visible on the mount of my filter.

facuet filter and aerator

If the faucet and filter are both 55/64", and yet the two are incompatible, I have no idea how to fix this, because surely there aren't any 55/64"-27 <-> 55/64"-27 adapters out there. I suppose I could try joining two adapters together, but it would probably make the end of the faucet too low.

Edit, possibilities:

  1. Threading of faucet is something like 24 per inch but same width, hence allowing me to screw on the filter, at least partially, but prevents a good seal.
  2. Width is slightly different (not sure if this is possible)
  3. Somehow faucet having too many threads prevents attachment of my filter

In the case of the first possibility, what thread size is the same width as 55/64"-27 M, but has <27 threads per inch?

  • Do you now what cross threading is, and are you QUITE sure you're not? It's a very common issue with these fine-thread but relatively large diameter fittings - easy to get them started wrong. Having the large lump of a filter on one side of the filter fitting will make it more difficult to get on straight, in my opinion.
    – Ecnerwal
    Dec 25, 2020 at 1:12
  • I've tried several times over a few days with the same outcome. I'm pretty sure things are aligned, and this filter has been successfully mounted on two previous faucets.
    – bernie
    Dec 25, 2020 at 1:17
  • I have an example of male 55/64"-27 threading (from an adapter), and the more I compare it side-by-side with the threading on the faucet, the more I'm inclined to conclude that the threading of the faucet is less dense than 27 threads per inch. But I have no idea what the threading of the faucet could be.
    – bernie
    Dec 25, 2020 at 1:24
  • 1
    13/16-24 and 13/16-20 both seem to be common enough, and are close (13/16==52/64) to the same size. See also: diy.stackexchange.com/q/109912/18078
    – Ecnerwal
    Dec 25, 2020 at 1:31
  • I did not realize 13/16 was so close to 55/64. I bet 13/16 is the faucet size. I wonder if there's a way to tell between 20 and 24. I'll have to bring the aerator to the hardware store and hope they have one of those thread gauges. It's probably 13/16-24 if I were to guess since it can partially slide into 55/64"-27.
    – bernie
    Dec 25, 2020 at 2:32

1 Answer 1


It turned out that the faucet end and the filter were both 55/64-27. I was able to confirm this when 13/16 and 15/16 both did not fit. The lesson is that if it screws on, it's probably the same size. The various faucet sizes are different enough that they won't be able to thread onto each other.

But why was the filter not fitting well despite the threading compatibility? I studied my filter closely. You thread it onto a faucet by turning a metal ring. It's my belief that the ring needs some sort of flange on the faucet to brace against in order to seal correctly. As you can see in the original pictures, th current faucet head consists only of threads.

The solution I took was to use two adapters. First, a 55/64-27 F to 15/16-24 F adapter onto the faucet. Then, a 15/16-24 M to 55/64-27 M adapter onto the first adapter. Then finally the filter. In the picture below, the M/M adapter is not really visible, but it connects the filter to the first adapter. Prior to buying the 55/64-27 F to 15/16-24 F adapter, I thought that I might be able to thread the adapter in reverse (the larger side facing up) so that it could provide a flange on the faucet that I could twist the filter's ring against, but that did not work; not enough room.


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