Stop tap - what thread is this?

grumpy_codger

I'm rearranging some plumbing and want to simplify this mess with adapters.

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It's my indoor water meter disconnected from the stop tap. The meter's thread is 3/4" BSP (the same as on appliance valves). 

Currently it's stop tap >> 22mm pipe >> 22mm to 15mm adapter >> 15mm to 3/4" BSP female

However, I can't figure out what thread it is on the stop tap:

  

It's definitely not 3/4 BSP - the diameter is slightly bigger, the pitch is smaller. And it's not 22mm compression fittings thread.

The stop tap terminates 27mm black MDPE pipe. And before you ask, unfortunately I don't have a personal outdoor stop tap, so replacing this one is problematic.

The house is 1985.

Any experts here?

Section62

grumpy_codger said:

.... 

It's definitely not 3/4 BSP - the diameter is slightly bigger, the pitch is smaller. And it's not 22mm compression fittings thread.

The stop tap terminates 27mm black MDPE pipe. And before you ask, unfortunately I don't have a personal outdoor stop tap, so replacing this one is problematic.

The house is 1985.

Any experts here?

Some compression fittings were made with a much finer thread... not 100% sure, but it might be called 'British Brass Thread'?  Or else possibly manufacturer specific.

I've thrown a lot away over the years, but still have a few somewhere.

If you have the matching compression nut on that assembly then could you reuse it... although you'll probably need to find a short length of 3/4 pipe and buy a 3/4 olive.

QrizB

Is it old 3/4" compression? Was still hanging around in the 80s.

Does the piece you've unscrewed have an olive inside?

grumpy_codger

QrizB said:

Is it old 3/4" compression? Was still hanging around in the 80s.

Does the piece you've unscrewed have an olive inside?

Yes, it's a short piece of 22mm pipe with two nuts and two olives. I think one of the nuts came with the tap. 

The bit at the top is usual 22mm compression fitting 22mm to 15mm adapter.

WIAWSNB

Well, it's definitely 3/4. 'Cos the tap body says so. 
Is there a manufacturer's make on the other side?

Since you have the nut that fits, there's no good reason to not reuse that. Worth taking the whole short pipe with nuts to a decent plumber's merchant nearby, and ask them about reusing the nut but with an olive to suit 22mm pipe - you should then be good to go in metric.

Give it all a good clean, and then a smear of potable-water pipe-jointing compound.

Section62

WIAWSNB said:

...
Since you have a nut that fits, is it worth just taking this to a decent plumber's merchant nearby?

Doubtful they would sell them anymore, probably little demand - if the nut isn't damaged then a plumber would just reuse it, or if the nut is damaged then just replace the stopvalve (noting this isn't so easy with no external stopvalve).

The main issue is if this fitting was designed for 3/4 pipe then the nut and the socket might be undersize to accept 22mm pipe (some are, some aren't).  If so, some 3/4 pipe would be needed to reuse the fitting, or a reducing tool to squeeze the 22mm down to 3/4.

But as grumpy_codger says the existing pipe is 22mm, then unless the nut is damaged the only thing needed should be a replacement 22mm olive.

grumpy_codger

No make, nothing. Possibly, QrizB is right and it really is old 3/4" compression thread. The tap is designed for 3/4" pipe as 22mm one fits nicely into the tap. 
I think I'll cut the short 22mm pipe with nuts and then use the nut and some 22mm to 3/4 BSP female adapter thus excluding 15mm step. Possibly 22mm bend as the plan is to fix the meter horizontally. Or just will use 22mm elbow instead of the current barrel. 

WIAWSNB

grumpy_codger said:

  

3/4" NPT is fractionally larger in diameter than BSP, and also has more threads per inch. A possibility?

But, if the nut is in good condition, I'd just reuse it with a metric-suitable new olive. 

Does that brass thread even have a small taper? 

FreeBear

WIAWSNB said:

grumpy_codger said:

  

3/4" NPT is fractionally larger in diameter than BSP, and also has more threads per inch. A possibility?

But, if the nut is in good condition, I'd just reuse it with a metric-suitable new olive. 

Does that brass thread even have a small taper?

3/4NTP and 3/4BSP are both 14 threads per inch. NPT is (nominally) 0.23mm larger, and the thread form is slightly different (60° as opposed to the 55° for BSP). M26x1.5 is another thread sometimes found on fittings, but we can rule this one out.

If 22mm pipe is a snug fit in the stop tap, I'd suggest reusing the old nut with a new olive. Preferably a copper one as they are slightly softer than brass. Resist the temptation to overtighten the nut. Compression fittings only need to be done up hand tight and then given quarter to half a turn with a well fitting spanner.