Confused about flexible connectors and sizing - please help!

[slowman]

Hi folks

First, I'm not a plumber by any stretch of the imagination and I'm working things out as I go along. I've been having problems with the valve in my toilet cistern. I have bought one of these to replace it with:

http://www.screwfix.com/p/fluidmaster-bottom-entry-fill-valve-uk/84007

There was copper fixed right into the bottom of the old inlet valve and I decided to replace that with a flexible hose to make sure I could get a good fit to the new inlet valve. There is an isolation valve in the copper supply already, so I bought a bit of pipe, some olives, PTFE, pipe cutters and fibre washers.

The copper supply is 15mm. I measured the diameter of the Fluidmaster shank and it is 3/4 inch or 18mm. So I looked on Screwfix and I bought one of these:

http://www.screwfix.com/p/wras-hose-15mm-x-3-4in-x-1000mm-x-10mm/31385

Because it says 15mm and 3/4 inch. Everything's fine except the nut on the flexible hose is actually ONE INCH diameter. It says on the packet 3/4 inch and it gives instructions for connecting to a cistern. So I am confused. I have, perhaps stupidly, assumed that the 3/4 inch refers to the nut size, since the 15mm refers to the pipe fitting end size, the 1000mm refers to the hose length and the 10mm refers to the hose bore, which is all logical.

MY QUESTION: Have I by a million to one got a flexi hose with the wrong size nut in the wrong size packet? Or do I need to in fact get a 1/2 inch flexi hose to fit a 3/4inch diameter valve shank? If so, can you explain why that is? Or have I just bought the wrong kind of flexi hose altogether? If so, can you point me at the right one?

I hope someone can help me out fast, as my toilet is out of action and all the DIY stores are now shut as it is Sunday after 4pm. Unless anyone knows where you can get emergency plumbing supplies in Bristol on a Sunday evening??[/img]

 

[Dan Robinson]

You bought the wrong one. You need a ½" t0 15mm.

It is the thread you measure not the nut.

http://www.screwfix.com/p/wras-hose-15mm-x-1-2in-x-1000mm-x-10mm/68187

 

[Jamesie]

Just a little tip, if you havent got a service valve as part of the suply to the cistern, fit one in. Should you ever need to remove the toilet you can still keep the water on in the rest of the house.

 

[slowman]

You bought the wrong one. You need a ½" t0 15mm.

It is the thread you measure not the nut.

http://www.screwfix.com/p/wras-hose-15mm-x-1-2in-x-1000mm-x-10mm/68187[/QUOTE]


Hi Dan - thanks for your reply. I had a feeling that was going to be the case. However, the shank is 3/4 diameter, so how do you know you need a 1/2 (thread) nut? How come the diameter of the nut is not specified? Does a 3/4 diameter always mean a 1/2 thread?

 

[slowman]

You bought the wrong one. You need a ½" t0 15mm.

It is the thread you measure not the nut.

http://www.screwfix.com/p/wras-hose-15mm-x-1-2in-x-1000mm-x-10mm/68187[/QUOTE]


Hi Dan - thanks for your reply. I had a feeling that was going to be the case. However, the shank is 3/4 diameter, so how do you know you need a 1/2 (thread) nut? How come the diameter of the nut is not specified? Does a 3/4 diameter always mean a 1/2 thread?

I even looked here: http://en.wikipedia.org/wiki/British_Standard_Pipe and the 1/2 inch BSP thread says is corresponds to a 21.3mm OD (outer diameter right?) rather than a 18mm OD - so I still wasn't sure. Can you enlighten me?

 

[Dan Robinson]

WC valves are either ½" or ⅜".

Whatever you are measuring it is the wrong thing.

 

[slowman]

WC valves are either ½" or ⅜".

Whatever you are measuring it is the wrong thing.

Hi Dan

I understand that I need a 1/2 inch nut, thanks for your help - I'm just trying to understand what the relationship is between the thread and the diameter of the shank. To look at it in terms of thread then, to decide if you want 1/2inch or 3/4 thread, how do you measure the thread exactly?

 

[denso13]

"BSP parallel fittings and their specifications can be identified by a completing few calculations.
To find the thread size:
1) Measure the O.D (outer diameter) of the BSP thread.
2) Take the O.D measurement (in inches) and subtract 1/4 inch (.25”)"

http://www.pipefittingsdirect.co.uk/contents/en-uk/BSP Identification.pdf

Therefore, your thread measured ¾", take away ¼" which gives the BSP size - ½"

 

[slowman]

"BSP parallel fittings and their specifications can be identified by a completing few calculations.
To find the thread size:
1) Measure the O.D (outer diameter) of the BSP thread.
2) Take the O.D measurement (in inches) and subtract 1/4 inch (.25”)"

http://www.pipefittingsdirect.co.uk/contents/en-uk/BSP Identification.pdf

Therefore, your thread measured ¾", take away ¼" which gives the BSP size - ½"

Hi - well that's great to know - still don't understand why, but thanks for the tip.

 

[Whitespirit66]

Jeez. You've really complicated a straightforward job, but I suppose if it's your first time, you're still learning.

You really should have left the old copper pipe on, and just put a new 1/2" fibre washer in the joint to the new valve. Connect new valve, then Bob's your aunty's live in lover.

As you're going flexi route, buy a 1/2" BSP tap connector X 15mm compression.

And I can guarantee there aren't any late Sunday emergency plumbers' merchants in Bristol. Even Been & Queued closed at 4pm. Just throw a bucket of water down, maybe two for sit-downs, after using the khazi 'til the morrow.

 

[Charnwood]

still don't understand why, but thanks for the tip.

It works because thread pipe is measured by ID - Inside Diameter, therefore measuring the outside diameter of the thread or the inside of a nut will not give the nominal size of the pipe or fitting required.

Nominal: (of a quantity or dimension) stated or expressed but not necessarily corresponding exactly to the real value.

Is there a reason you ignored the 1/2" stated on the Screwfix website?