On Thu, 08 Sep 2005 16:23:18 +0100, Deville wrote:

Should I use plumber's tape for the threads?

You certainly can, but it's probably not needed or especially useful.

There are two sorts of thread: parallel and tapered. Both work in the
same way - they use the lengthways wedging action of a screwthread to
squash some sort of compressible gasket. The difference is that a
tapered thread can seal against the thread itself, but the parallel
thread must seal against some separate flat radial face instead - either
the end of the threaded tube (inside the thread) or some sort of
flat-faced nut (outside the thread). Usually a tap will seal with a
small washer on the end of the threaded tube, a bulkhead connector in a
cistern (toilet or loft) will use the nut and external washer.

On a tapered thread there's obviously no washer (and in fact an added
washer will be no benefit at all). However threads alone aren't a
perfect match, so thin PTFE tape is a useful sealing method.

On the parallel thread, you need the washer to seal. Stick what you like
on the threads, they'll just never get the compressive force to make a
good seal there. The same principle applies to compression fittings -
it's the olive that matters, not the nut.

You _might_ use PTFE tape on a parallel thread as an anti-seize
mechanism, but it's likely to be scrubbed off as much as it stays
behind.