tap connector /ball o valve conection to cistern valve

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Having trouble getting a bottom inlet cistern valve (low level cistern , plastic valve) to seal on to tap connector with integral service valve.
The fibre washer is fitted to tap connector .
Could it be defective fibre washer problem?
Do I need a little ptfe tape or paste to seal the connetor on to plastic thread?
Worried about over tightening the tap connector since I have already ruined two ball crane valve threads on this project. due to bad alignment and /or posibly overtightening.
The valve itself and backnut is sealed to cistern ok
On most recent and third attempt time I installed a fluid master quiet valve which has a fairly robust plastic thread compared with the soft plastic type of other valves.
I am using rigid (chrome ) pipework.
I have fitted a few WC`s before so this is a new leak diagnosis for me.
This is a new toilet and new pipework.

[plumbbob]

This problem is a real nuisance, and is caused by the pipe you are connecting is not perfectly on line with the valve fitting and of course you cannot crank it up because it is only a plastic fitting. Frustrating isn't it!

I always work backwards and fit the connector first then fit the pipework to suit.

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[quote="plumbbob"]This problem is a real nuisance, and is caused by the pipe you are connecting is not perfectly on line with the valve fitting and of course you cannot crank it up because it is only a plastic fitting. Frustrating isn't it!

I always work backwards and fit the connector first then fit the pipework to suit.[/quote]
Thanks for that, I did on the third attempt, dismantle the fittings and fit
the tap connector first and the result was improved but not 100% sealed.
The pipework is a bit tight and it is reassuring to have your experience and comments on the importance of alignment. Looks like a check, remeasure and remake as required.

[htg engineer]

Try fitting the tap connector with fibre washer to the cistern first. Then put into position and slide pipe in.

Sometimes in tight awkward places you cannot see if the fitting is perfectly square. Get it wrong and you're knackered as the plastic with strip easily.

If still no luck, get a flexi pipe that has a 15mm nut and a rubber 'o' ring - like the ones used for combi fills. http://www.screwfix.com/search.do;jsess ... combi+fill
not ideal, but with a toilet if the pressure is reduced it wont be noticeable.

You could try other flexi hoses, But with this one the female part pushes into the plastic pipe whereas others rely on the rubber washer sealing between the pipe and the hose, If the threads are dodgy or not square it'll never seal.
Worth a try.

Htg