I recently did some plumbing work in my house which involved using threaded (NPT) black iron fittings in the 1" and 2" sizes.

At the outset I was concerned that I could overtighten them and crack the female side. But in a few cases I had to really torque down to get the right orientation and had no problems. To get an appropriate number of threads turned in (based on what I've read) they really required a lot of force! In the 2" size it was extremely tough using 18" wrenches to do the last 1/4 turn or so.

In contrast a few copper NPT/sweat adapters definitely seemed malleable and I held off more on those.

In all cases I checked threads, wire brushed, etc. so the fittings should have gone together without impediment.

Is breaking a black iron fitting actually possible / likely? Assume you are using an appropriate sized wrench and don't have superhuman strength.

Note - in my specific situation I used teflon tape coated with Rectorseal for these joints. (Using both was based on advice from lengthy internet "research". This might have been "belt and suspenders" but has worked perfectly.)

But this question is meant to be more broad then just that specific case, so feel free to answer considering different joint preparations if that is appropriate! Thanks

  • @isherwood I agree of course but I wasn't sure why you had asked about that? I assume it is pertinent but just not clear specifically how. Do in infer correctly that it is a question about lubrication? Jan 18, 2019 at 15:50
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    Torque values (and the related effort) are affected dramatically by what (if anything) lubricates the threads. As d.george mentioned, lubricated cast-iron parts can break more easily since the fitting can more easily be tightened.
    – isherwood
    Jan 18, 2019 at 16:06

1 Answer 1


Yes, you can break cast iron fittings from over-tightening but you rarely do. That said, if you use teflon tape on cast iron fittings, they tend to break more often. Years ago, I learned that the use of teflon tape for the threads of an oil burner pump will easily break the pump's casting. (found that out on a very expensive pump). My only question is; are you sure that you have cast iron fittings and not black iron fittings (malleable iron) which are more common?

  • Question is updated about cast vs. black - if you can add any more info on how the two are differentiated with regards to breakage that would be v interesting! Jan 18, 2019 at 15:52
  • Cast iron is a very hard almost non stretchable metal that in my line of work was used for steam lines and condensate lines that had to resist a caustic environment. Malleable iron fittings (black iron as they were called) were a softer metal that could easily stretch without breaking.. Blach iron or malleable fittings are sold everywhere and are used for gas lines and the like. Also, cast iron fittings can crack from a hard blow where malleable iron will usually bend.
    – d.george
    Jan 18, 2019 at 16:21

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