"Andy Hall" wrote in message
...
On 2007-07-10 08:10:58 +0100, "DJB" said:

I've fitted a new immersion cylinder but am having a problem with the
drain-off valve. When I bought the valve I thought it should have a
fibre
washer, but a friendly plumber told me that it doesn't and I should
instead
use 13 turns of PTFE tape. The problem is that even though I've tried
this
a few times I can't get the damn thing to point downwards - and it leaks!
(the irony - having to use the drain-off valve for the first time because
IT
leaks... grrrrr.) To me it doesn't seem right: the face of the valve
seems
smaller than the face of the outlet on the cylinder as if something else
should be there. Searching the archives has left me thinking that this
is a
1/2" BSP parallel thread and that what the plumber told me is probably
correct, but it still seems odd (to my naïve plumbing skills) that the
immersion element came with a nice washer but the valve didn't. I'm
going
to try again this morning with even more tape, but thanks if anyone can
put
me out of my misery and tell me if I'm doing anything wrong. I've put a
picture up he
http://myweb.tiscali.co.uk/dbecker/drain-off-valve.jpg
David

He's right, these should just screw in and have PTFE tape - no fibre
washer.

One thing that can happen on some fittings - some cylinders and radiators
are bad for this - is that the thread tolerance is a bit poor to the
extent that the fitting is rather loose.

This suggests use of a lot of turns of PTFE tape (like his suggested 13),
but sometimes the tape is then cut by the thread or bunches at one end and
the fitting leaks.

I've found two solutions that generally work well.

- Get some PTFE tape for gas. This comes on a yellow reel and is thicker
than the mimsy stuff for water. Try about 4-5 turns or so on the
fitting and screw in by hand. It should be possible to turn the fitting
in your fingers with a certain amount of effort but it should not require
a wrench until final tightening and should not be so loose that it turns
easily. If either then remove and apply more or less tape as
appropriate.

- Get some Boss Green and some hemp. Apply a small amount of the sealing
compound then wrap some hemp onto the threads (right way round as for
tape) and finally another smear of compound.

Both should work well and you should not have a leak.


Thanks both of you for the fabulously quick replies, I'm off to the
plumber's merchants now :-)