Originally Posted by JimVonBaden
As sugested above, I will be looking for just the top part, but may not be sucessful.
I am also considering making and fitting a collar for the ridge around the female threads. If I can get a tight fit, and use some epoxy, it may solve the issue as well, and much cheaper.
As for the metal insert, it is identical to the plastic one, which may be the reason even the plastic ones fail. There appears to be some pressure put on the raised female portion by the ridge on the insert. Adding a small spacer moves the pressure to the top of the raised female threaded portion, removing the outward stress on the ridge, and forcing the sealing part to the threads. Not ideal, as the insert seems to want to seal on the contact point, think oil drain plug with a crush washer.
Speaking of that, maybe a washer made of semi-hard plastic would be a better spacer than a flat washer? I'll look into that as well.
I'll get some pictures to illustrate what I am thinking soon.
This is an unusual application of a pipe thread. As you know, pipe threads are tapered and as you tighten the joint the threads pull the tapers together forming the seal. The wedging action is what creates the high stress in the female collar. What's wierd is that the fitting tightens down all the way to where the hex "nut" face contacts the flange collar. A typical pipe thread joint will leave one or more exposed male threads. This ensures that the tapers are properly loaded and NOT the nut face. You don't want both.
By adding a washer, but you will lose the seal provided by the taper. You'll be relying on a thick layer of thread sealant. May work ok.
Like someone mentioned above, a straight thread with an o-ring would have been better.
The bigger issue may be the continued material degradation over time. The loads and stress on your flange haven't changed, but the cracks extended. This may be a warning sign, i.e., leak before break.