leaking radiator joint

No

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!

Loctite 55 PTFE string.

Seconded.

I've used it for an immersion heater to cylinder, and a very damaged

You do need a lot of PTFE tape. 8 - 10 turns sometimes. Plumbers Mait is wonderful stuff for cistern/pan joints, waste pipes etc.

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.

What's the difference between this and ordinary PTFE tape that has been twizzled (technical term!) into a thread?

Cheers.

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.

Ahh! Thank you very much. I shall have to invest in some of this wonder material!

Cheers.

Everything you ever wanted to know is here:

and "telfon"

Many thanks. Looks like Loctite 55 is the one for me.