Help joining to 1/2 inch i/d plast pipe

[Matt]

Hi all,

Im trying to extend the existing fresh water system in our narrowboat. All the existing pipes are grey plastic, with an outer circumference of

40mm, which works out as an inner diameter of 1/2 inch (12-13mm). I dont really understand this, as I thought imperial was worked out on the outer diameter...

Anyway, the new system will all use 15mm pipe. This needs to be connected in by cutting an existing pipe and joing to both ends of the severed pipe. Whats the best way to do this? I have asked around the plumbing shops, and the only thing they can suggest is to use a copper coupler (15mm) and just do it up very tight on the old pipe (and hope the olive crushes down enough).

Any help very much appreciated, Cheers

[NB Harry]

Sorry, meant plastIC pipe in the title

[Set Square]

At first reading, I thought you had written about pipe with an OD of 40mm and ID of 13mm - which would have rather thick walls(!) - then I realised it was the *circumference* which is 40mm!

According to me calcs, a 40mm circumference gives an OD of 12.7mm - which is pretty close to 1/2". If this is the case, the ID *can't* be 13mm - it's got to be less.

[As an aside, imperial pipe sizes are based on the nominal ID - so 1/2" copper pipe has an ID of around 12.7mm, and an OD pretty close to 15mm. However 1/2" iron pipe has an OD pretty close to 3/4"]

Having said all that, the pipe in your boat doesn't seem to correspond in size with anything used in normal domestic plumbing. You may be able to find a fluted hose connector which would be a tight fit inside the plastic pipe - with a standard fitting - maybe 3/8" BSP? - at the other end - which you could then join to 15mm copper pipe with suitable adapters.

[NB Harry]

Hi Set Square,

Sorry, got my i/d and o/d's muddled. Meant to say 13mm outer diameter, and not 13mm inner diameter as I wrote.

Any pointers of where to look for the hose connector? I have tried a few local plumbing shops and no joy with anything more likely looking than a standard coupler.

Cheers

[Set Square]

You'll probably have to go to a hydraulics company. To see the sort of thing I have in mind, go to

and click Guest Login. Then, in the drop-down window, click on Fittings Catalogue, and then on Push-on Host Fittings.

Have a look and see whether any of those would be any good. You can probably get something locally once you know what you're looking for.

[John Rumm]

It sounds like a compression fitting will probably do it.

If not one (somewhat cludgy) way that ought to work would be using a pair of MDPE transition fittings back to back with a short bit of MDPE between them. See BES product code 13544

for an example.

These are designed to connect 25mm MDPE water pipe to just about anything (lead, copper, plastic, iron etc) and cope with a wide range of outside pipe diameters.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember [e-mail] saying something like:

Try a 15mm straight compression connector for size. At the end where you have to join on to your old pipe, see if a 1/2" olive fits. If it does, use a 1/2" olive and nut on that end of the 15mm connector.