I'm trying to plumb up an old cast iron radiator. I've got 2 new 1
1/2" threaded inserts which screw into the body of the rad. Into these go the valves.
I've used PTFE tape on the threads but don't seem to be getting a sealed joint on one end. Is there a technique for this? Alternatively there's a cement called plumber's mait, which I think might be the sealant originally used on cast iron rads.
Try getting hold of some hemp fibre and a small pot of BOSS White or Hawk White. Wrap the hemp fibre into the thread and rub on some of the white. Screw into the rad until tight. If any moisture weeps by the join it soaks into the hemp which swells and forms a seal. Traditional ways are often the best when threaded joints leak. Just don't get white onto immersion heater threads as you will have no end of problems in removing the heater when it fails!
I assume that the leak is on the 1 1/2" thread where the reducer screws into the rad? This is most likely due to the threads in the rad being grotty - probably with hardened residues of old Boss White, which are tearing the PTFE tape.
You really need to clean up the treads with a 1 1/2"BSP tap. If you don't have one (I'd be surprised if you *did*!), you can make an acceptable substitute using an old male fitting and cutting some slots at right angles to the threads with a Dremel or equivalent.
Failing that, you could use gas-grade PTFE tape which is a lot tougher than the normal stuff. Or you could even resort to Boss White and hemp - which is probably what was used on the radiator in it's former life.
It's white synthetic fibre, not ptfe - I think. It's more like an ultra-heavy-duty dental floss (more like the thickness of lacing cord if you're familiar with that), with what feels like a waxy coating on it. Let's call it dental floss for alligators.
I suspect because it's manufactured to be a cord, it has a reduced tendency to chew up when being used on a rather rough or damaged thread, and withstands quite a bit heavier mechanical loads. It's certainly not a cord you can stretch or break between your fingers, the box has a little dental-floss style cutter on it.
However the *practical* difference is that a damaged thread that I had unsuccessfully tried to seal several times with tape (eventually going as thick as I could - still without eliminating a very slight weep) - sealed first time with this stuff.
And that or similar seems to be a common experience.
Considering the hassle of re-pressuring and still finding a slight weep (which maybe you can cure, maybe you can't), I suspect many people will spend a little more if they're likely to get a first time perfect seal.
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