We've had a lot of sediment in the water after this past month's flooding. Water pressure also jumped to 100psi. The Water Authority had to shut down a plant and reroute the system through some disused conduits. So I bought an overpressure valve, a little spin-down filter, and a two-stage point-of-entry filtration device and am now working through what fittings will best enable individual components to be removed and serviced.

The plumbing in the house is 3/4" copper. The spin-down filter has multiple ways to connect. It has 1" MNPT threads. Inside each of those threads is a 3/4" FNPT thread. There is also a threaded coupling option that fits over the 1" male thread; it has a 3/4 MNPT brass fitting that emerges from the coupling's hex nut; those 3/4 MNPT brass fittings would protrude on either side of the filter like ears. Inside the coupling's hex nut the seal is made with a flexible washer.

The 3/4 MNPT brass fitting has no wrench flats (see picture) but there are two little tabs inside it. I assume those tabs are meant to be used with a key wrench of some kind, to let you tighten a 3/4 FNPT fitting onto the brass fitting? If that's right, is there a name for that kind of key wrench?

brass fitting without wrench flats spin down filter with fittings

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    I believe you'll find the outside threads are not pipe threads (tapered) but rather straight threads (for the included union fittings) sanitaryfittings.us/npt-vs-nps I'd also check to see if you could sweat copper tubing to the inside of the outside-threaded union fittings. They have that look to them.
    – Ecnerwal
    Commented Sep 26, 2021 at 15:54
  • @Ecnerwal : Here is a video showing PEX and Sharkbite-style connections. They want you to use the handle of the supplied wrench to tighten this. But on another matter, the direction-of-spin of the union's nut makes no sense to me around the 30-35 second mark. amazon.com/SimPure-Sediment-Reusable-Spindown-Pre-Filter/dp/…
    – mr blint
    Commented Sep 26, 2021 at 18:38

2 Answers 2


To unscrew that fitting with the inside knobs or as you said "tabs", try using a "spud wrench" which is a tool used by HVAC people to replace union fittings on cast iron radiators. Your local HVAC company may have one you could borrow for a few minutes.


Well, I have found my socket set had a wide blade that fitted some, but even some large allen keys can fit.

Or cut some flat bar stock and bend for a handle. Lots of options really as it does not need to be stupidly tight, especially as some ptfe would be appropriate.

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    And not buy a special tool I'll only ever use once? Are you crazy, man? :-)
    – mr blint
    Commented Sep 26, 2021 at 15:15
  • @mrblint so ptfe was named for you: Pull Tight (it’s) F***** Expensive. :)
    – Solar Mike
    Commented Sep 26, 2021 at 15:49
  • I like your ideas, @mrblint, and would like to subscribe to your newsletter! :D
    – FreeMan
    Commented Sep 27, 2021 at 14:02

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