On 28 Mar, 19:32, andyv wrote:
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.

Is this any good?

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!