Fitting new siphon in low-level toilet cistern?

[Lobster]

I'm scratching my head a bit here. Our downstairs khazi has a slimline, low-level cistern with a push-button flush which some family members find very hard to operate, as it requires quite a substantial downward force. Nothing actually wrong with it though; and I currently have the cistern in bits with the intention of replacing the siphon with a flap valve

This fits in place OK, however my problem is attaching the flush pipe to the bottom of the cistern, to connect it to the pan. Formerly, the cistern had a single threaded collar with an inbuilt rubber seal, intended to bear against the bottom of the siphon as it protruded out the base of the cistern. Tightening this collar both sealed the siphon to the base of the cistern, and the flush pipe to the siphon.

Problem is that with the new flap-valve, the original collar is slightly too large (different thread); whereas the new threaded collar which comes with the flap valve does not include a rubber seal, and presumably is not intended to seal it to the flush pipe. I get the impression this flap valve might be intended for use only with close-coupled cisterns? The existing flush pipe does fit snugly into the base of the flap valve, and it would probably be possible to effect a seal using lots of silicone but I really don't want to have to go down that road!

I'm a bit hampered by the fact that the other toilets here are close-coupled, so I don't have any other flush pipes to examine, but does my original system sound 'normal'? The single threaded collar sounds odd to me. I would just swap out the whole cistern but for the fact that it's an old coloured one (turquoise!!), which vile though it is, matches the pan, basin and tiles...

Anyone know what I'm doing wrong, please?

[Roger Mills]

Blimey, I've never seen a flush valve like that. But it looks as if it's designed for close-coupled cisterns. There's a flange and rubber washer on the inside and a big plastic nut on the outside. There's a bit of thread sticking out after the nut is done up, and you would normally put the doughnut round that.

I would have expected all flush valves - of whatever type - to use standard BSP threads, so I'm surprised that your old fitting to attach the flush pipe doesn't fit. How much too big is it, and how different is the thread. Could it be persuaded to tighten up if you wrapped lots of PTFE tape round the threaded bit?

It might just e woth taking the flap valve and the flush pipe to a plumber's merchant to see whether they have a different type of nut which would fit.

[David Lang]

Similar to the way a compression waste fitting works?

I think you might be right. I've never come across one in a low level cistern.

You don't!

I think Rogers idea of taking it to a proper plumbers merchant is the way to go.

[Lobster]

On 07 Apr 2016, David Lang grunted:

Exactly.

That said, I've just noticed that the instructions talk about disconnecting and refitting the flush pipe, so presumably not! (see

or l et.pdf

It's odd though that although the length of the threaded outlet of the flap valve is the same as on the old siphon job, due to the effective depth of the base of the cistern (a whopping 35mm due to a reinforcing circular rib around the outlet hole) once the single threaded collar is applied there is no remaining thread exposed anyway, for a second sealing collar to be attached.

Yeah, maybe so.

FWIW I've also found a supplier of spares for the original cistern (of no direct use to me as mine isn't actually faulty, but there's a picture of the beast here:

and the siphon here: (you can see that although the i.d. of the threaded outlet is a standard 1.5", the walls are much thicker there than in a 1.5" compression fitting, so the thread size is not standard. (Roger - the difference is far too much to fill with PTFE!)