Too much PTFE?

[bungeejumper]

Don't laugh, I've just cracked an old brass tap assembly in the garden by using too much PTFE on the threads. Or at least, that's what the community chat at Screwfix seems to think can happen. I must be stronger than I thought.

Who'd a thunk it? All I gave it was about seven turns of PTFE (duhh, clockwise I think, I can never remember the right direction), and when the tap was tightened up to the brass back plate (https://cdn.aws.toolstation.com/images/ ... /74070.jpg), it spouted water everywhere. Looking closely at the back plate, I saw a shiny new crack that extended for almost a centimetre into the threaded part of the plate that accepts the tap. And it was almost a millimetre wide. How the hell did I manage that?

Fortunately this isn't a mains-pressure tap - it just connects to my rainwater butts, so it's only carrying about four feet of head. It's in a frost-free location beside the compost heap, which is always warmer than elsewhere. Maybe it was just a faulty component from the start?

So, serious question. How much PTFE do you use? Some say three turns, some say ten. In the meantime, Mr Amazon is proving his worth by getting a new tap and backplate to me this morning. It could have been worse.

BJ

[DrFfybes]

Depends on the quality of the fitting, some are cast poorly and need a bit more,but generally 4-6 turns, in the same direction as the nut tightens.

How long a spanner did you use to tighten it, and how big screws to mount the back plate? I'd suggest the old back plate might have been weakend over the years, or you got a bit of grit in there.

Paul

[bungeejumper]

How long a spanner did you use to tighten it?

Spanner? That's finger tight, mate. There are some advantages to being descended from a long line of blacksmiths, you know.

Seriously, a puny eight inches. (Quiet at the back, there.) And a stilson wrench, because it was the first thing that came to hand. For serious applications, I also have a two foot long navy wrench that came from the engine room of a WWII destroyer. If Hitler couldn't beat it, a bit of crappy Indian brass casting would have much of a chance.

BJ

[Urbandreamer]

Well...

I would use 2-3 turns, or about 1 layer.

However in theory you don't need to use any. The thread form and thread change due to taper are designed so that they should produce a seal themselves.
in practice we tend to use PTFE/Teflon tape. The non-stick properties ease the thread far enough to lock while tape in the wider part of the taper help with any weap.

Too much tape in itself wouldn't cause you to fracture the fitting. What it would do is move the point where a lock is achieved. Hence the tap might have to be at a funny angle when a lock is achieved. I suspect nobody would want that, leading to forcing the wedge further into the taper and splitting the fitting.

Ps gas threads predate Teflon/PTFE.

[88V8]

Depends how slack the thread is.
I plumbed in a new loo cistern recently, using the old brass valve, with no PTFE at all.

Usually 3-4 turns. Taps can be problematic, as one usually requires them to end up at a certain orientation, as opposed to just 'tight' and this can easily lead to overtightening.

Castings are more brittle in cold weather, or perhaps it's been overtightened or frost weakened in the past.

In the dim & distant, I was installing a reducing bush in a cast iron rad using a 24" stilson, and managed to split the rad.
Bad enough, as it was a 24" three-column rad of a certain length that I needed for the bathroom. Worse still, we had to get it downstairs for repair, and it was seriously heavy. Perhaps 200lb. The builders had carried it up, but we were in a hiatus awaiting the plumber, so no builders.
The inadvisable process of sliding it down the stairs on a scaffold board with me on a rope at the top as the only restraint, could have ended badly, but thirty years on I still remember the crash as it hit the floorboards on arrival at the bottom and my relief that it didn't erase the balusters, and the wife had managed to leap out of the way.

The repair leaked, had to scour the salvage yards for another iron rad.

V8

[Mike4]

Any suggestions specifying the number of turns are doomed to failure. The manufacturing tolerances in BSP threads are massive and the only way to know is to screw the two parts together and feel how baggy (or tight) the fit happens to be and then wind enough turns on to get a nice tight fit.

Additionally, PTFE tape comes in two thicknesses. Cheap and horrid PTFE is as thin as clingfilm and you'll usually need dozens if not hundreds of turns to build up enough material to really create some hydraulic pressure in the sealant to force it into the nooks and crannies of the threads as you wind it in. "GAS" PTFE however, is about 0.1mm thick (about the same as a sheet of copier paper) and anything from 3 to 10 turns is often sufficient to get a nice tight and secure joint.

Additionally additionally, with a bib cock you'll have a parallel thread with a flange at the base of the threaded stem so the real water sealing happens where this meets the outer face of the blackplate elbow. So the amount of PTFE you add affects the angular position of the tap as it tightens up.

Finally, I'd say your backplate elbow was a flawed casting and that stuff about adding too much PTFE on the other forum is rubbish. Brass fittings are strong enough to cope with enormous stress when tightening up. Splitting one is really rare and when it happens, you can feel it go in the way the fitting tightens in, or more accurately, doesn't tighten in as the fitting spreads instead.

[stewamax]

Use Loctite 55 thread instead of PTFE. Roughen the threads with file (yes really) and wrap diagonally. Especially good for radiator tails (whether tapered or straight fit) that are normally a PITA to seal with Boss White/Green or PTFE. It's extraordinary stuff.
For sealing washers and other unthreaded things I use Fernox LS-X (NB: keep the cap on except when actually using it - it goes off quickly in the tube)

Usual disclaimers.

[bungeejumper]

Footnote: The new tap arrived from Amazon (https://www.amazon.co.uk/gp/product/B07RXZ6P28/), and it went in smoothly. The on/off lever function is great for outdoor use, as opposed to a conventional turn handle with a washer. Happy bunny. Much thanks to all.

One query, though. This tap didn't come with a backflow check valve, which is a good thing from my point of view because the flow from my rain barrels is much better now. (And less susceptible to blockages from barrel gunk, I'm sure.) But wouldn't there have been a potential backflow issue if the tap had been connected to a mains supply instead? Does it make a difference that it was specifically advertised as a garden tap?

Thanks also to Mike4 for the heads-up about Gas PTFE tape, as distinct from the usual thinner stuff. Given that there isn't much of a price differential, would it be OK to use Gas PTFE in future for all water compression fittings?

BJ

[jfgw]

Another fan of Loctite 55.

Outdoor taps with integral double-check valves should not be used where they may freeze. The double-check valve should be a separate fitting indoors.

The only permitted exception is where an old tap that was installed before double-check valves were required is being replaced.

Julian F. G. W.

[Mike4]

Thanks also to Mike4 for the heads-up about Gas PTFE tape, as distinct from the usual thinner stuff. Given that there isn't much of a price differential, would it be OK to use Gas PTFE in future for all water compression fittings?

BJ

Yes its fine. I only ever buy gas PTFE tape and use it for everything, despite it costing 20p more than the ghastly ultra-thin stuff!