Trying to get a watertight seal between an outside tap (1/2") and brass fitting

[NY]

We have an outside tap fitting which consists of a 12 mm copper pipe soldered to a brass elbow that is screwed to the wall. This elbow has a standard 1/2" BSP thread (internal) into which a tap is screwed.

Ever since the original tap cracked when it froze last winter, I've had problems getting a watertight seal between the replacement tap and the brass fitting.

is a cross-section. The pipe and fitting are shown in black, and the tap thread is shown in dark blue (I've not shown the body of the tap which is to the RHS of the thread in my diagram).

The tap was provided with a rubber (not fibre) washer that butts between the end of the brass fitting and a flange on the tap. I have wrapped PTFE tape around the thread before screwing the tap into the fitting. The tap forms a watertight seal (no drip from the washer), but *only* if I screw the tap so tight that it points upwards rather than downwards ;-) If I slacken the tap

1/2 a turn so it points downwards, there is a drip from the washer.

How is the seal supposed to work? Does it rely entirely on the washer being sandwiched between the fitting and the flange on the tap, or is the tip of the tap thread supposed to form a seal against the narrowing in the fitting (on the LHS in my diagram)? Is there supposed to be a washer at the tip of the thread as well as one between the outer surface of the fitting and the tap collar?

Will I get a better seal if I use a fibre washer next to the collar, so it remains in place and doesn't get squeezed outwards as the tap is tightened?

It is a nuisance trying to get a good seal, because the builders who installed the outside tap during other building work did not fit an isolation valve inside the house, so the only isolation is the main stop tap for the whole house. If I'd noticed at the time, I'd have got them to fit one and provide access to it through the wooden board of a cupboard unit. So every time I want to test a new washer, I have to turn off the whole house and release the water pressure by opening a nearby sink tap, and then walk to the other end of the house to turn the water back on once I think I've fixed it.

[Clive Arthur]

<snip>

When I did this, I had to experiment a little with the amount of PTFE tape. Sounds like you need a bit more. Forget the rubber washer.

[NY]

OK. So it it purely the PTFE tape between the thread of the tap and the thread of the fitting which makes it watertight? I assumed that either the tip of the thread butting up against the narrowing within the fitting, or else the flange of the tap butting up against the outside of the fitting (with a washer in either case) was what made things watertight.

It's annoying because if I manage to get it right, I'll have to redo it next year after I remove the tap for the winter (and replace it temporarily with a 1/2" plug) and then refit the tap next year when freezing is no longer a problem. I really should have got the builder/plumber to fit an isolation valve for use in winter...

[Andy Burns]

Try some LSX?

[Fredxx]

If these are straight, and not tapered threads, then the intention is for the rubber washer to do the sealing. Stiffening up the threads with PTFE tape will reduce the pressure on the rubber washer. Best look at the quality of the mating surfaces. I would also use one method of sealing but not both.

I'm wondering if the elbow fitting is cracked? Perhaps use more permanent thread-locking fluid?

[The Natural Philosopher]

yiu have a couple of options

- lots more PTFE and forget the rubber washer

- add more (fibre) washer. Packs of washers are cheap to come by off ebay or punmb centre or screw'n'fux

[Tricky Dicky]

Seen a lot of plumbers using Loctite 55 sealing cord instead of PTFE tape and most seem to swear by it as being better and even saw one use it to fit a tap like you.

Richard

[jkn]

I agree with them FWIW - I got a much better seal, with the tap pointing the right way, with some of this sealing cord and a few practice tries. It's been good for 10+ years now.

J^n

[John Rumm]

Well first option. Take the base off the wall, turn the pipe through 180 degrees (elbow + a "street" elbow), and refit the other way up).

It sounds as if yours is supposed to work with the supply pipe coming up to the base, and the tap forming a water tight seal to the base with the O ring. Presumably all machined as a pair so that it reliably ends up at the right orientation when sealed.

You could dispense with the O ring and get a kit of fibre washers of various thicknesses. Experiment to get the right thickness of washers to allow a nice tight fit when it points in the required direction.

If you are forming a thread seal with PTFE, then it is designed to work with tapered thread fittings and not parallel thread ones. You can still make it work with parallel, but it takes a bit more effort. You will need to wind the PTFE in a taper, so that it gets thicker as it gets further away from the tip of the thread. You may need many overlapping turns at the thick end. (allowing the tape to fold as you wind it builds thickness more quickly). You should be able to feel it "tightening" as you screw the tap in - it needs to be a very snug fit on that last half turn). (PTFE string can be better for this)

Sounds like the most important job then is to fit an isolator. You can get ones designed to post fix that require little space or access (i.e. stick a hole saw through the panel hiding the pipe, fit the saddle to the pipe, and screw in the self cutting isolator.

The Aladdin ones are very good (pricy - but worth it for difficult access situations where doing it by more traditional means would be lots of time and effort)

The whole process:

[John J]

Stop wasting time with PTFE and rubber washers. Get yourself a tin of boss white and a roll of hemp fibre

[Roger Mills]

You beat me to it! That gives you much more control over getting it tight and with the tap pointing in the right direction at the same time. Even if the tap still moves a bit, it will go solid when the boss white sets.

[Robin]

If not worried about appearance IME there's no need for fibre on a joint that small: just splash it on all over* - though it is of course not meant for use with potable water.

*Henry Cooper accent optional

[SteveW]

Just use a liquid pipe thread sealant - no need for the washer to seal at all, no need to have it tight. It's the best option for any threaded fitting that has to end up pointing in the right direction.

[nothanks]

+1 (wool will do at a pinch)

[The Natural Philosopher]

I think "has to end up pointing in the right direction." is the key. That directly conflicts with using a sealing washer unless you are prepared to shim it.

Filling the threads with anything that works is the key. The old way is hemp and Boss white. PTFE is more modern, but its not a question of a few turns. Its a question of enough to mash up and utterly clog the threads until you can barely screw the tap on

[SteveW]

I found the liquid sealants back in the '90s, when we were sealing temporary, screwed, instrument connections, that were retaining gas at up to 210 Barg. One ring of it around the threads, screw in and leave for a couple of minutes.

PTFE tape was a no-no, due to the possibility of pieces becoming detached and getting into clean systems.

[John J]

During one previous life I had the dubious distinction of working for Planned Maintenance Engineering. One of the desk jockeys in head office issued and edict to all mobile engineers that to avoid risk of contamination of potable water systems boss white type materials and hemp must not be carried and any in works vans must be handed in for disposal. Instead ptfe fibre must be used to seal threads. On my next job I had to swap out a failed 3" heating circulation pump which required fitting screwed flanges to existing threaded pipe ends. Did the job. Opened the isolation valves and a drip appeared from the thread. With hemp it would have simply expanded and sealed in an hour or so. Not with ptfe fibre. Shut the isolation valves. Remade the joint, opened the isolation valves and drip appeared. After 4 attempts I went to a local plumbers merchant, bought a reel of hemp and a tin of boss white. Returned to site remade the joint with proper materials which sealed perfectly. I later proceeded to explain to the desk jockey he needed to actually go out and do jobs instead of sitting on his arse thinking up stupid ideas. His boss found his embarrassment hugely entertaining.

[Animal]

Rubber is at best a time limited seal on full mains pressure, not a great option. Use a fibre washer. Your fitting is designed to have the incoming pipe go down not up. Trying to get it to seal when not fully tightened is going to make life harder than it need be. PTFE is hard to get to seal on mains, though not impossible. Not recommended.

Polypropylene does the same job as ptfe but is much tougher. Low cost ropes are mostly flat polypropylene film scrunched up then twisted together. For thread sealing, either a proper setting thread seal or linseed window putty work ok. Epoxy is also an option if you glue it up & wait 24hrs before feeding it water. The upside is it's very easy to use & many households already have it. (fwiw Soldering the whole fitting might also work, depending on the metals involved.)

1st job is to fit an iso valve, that'll take the pressure off the rest, literally as well as figuratively.

[NY]

I imagine the brass blanking plug can withstand higher pressure from ice that forms in the 1/2" fitting than the thinner walls of a tap can. In theory the exposed copper elbow between the fitting and the pipe that goes through the wall is also vulnerable, but it seems to have survived.

It was the chrome-plated wall of the tap close to the ball valve (sphere with a cylindrical channel cut in it) that split.

Ideally I'd get a stop valve fitted, but the pipe within the house runs

*somewhere* behind a wooden panel that would need a whole cupboard unit removing to access and work out where to drill an access port.

The builders who fitted that cupboard unit and outside tap did not plan ahead for a stop valve.

[The Natural Philosopher]

the weakest point will always go.

In one particularly hard winter the *radiator * in my porch of the cottage I was renting split *inside the house*.

By making your tap more proof, you may simply split the copper pipe instead :-(

British winters seldom are hard enough to warrant the work. Probably with 'climate change' they will get worse and worse. Until we are in a new ice age.

Let's face it. whatever they forecast, generally the opposite happens...