Can't screw backnut to the lockshield valve

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Hello all. I am trying to install a radiator and I am stuck in the last step. So, for whatever reason, I can't tighten the backnut with the olive nut and the lockshield valve. I can't figure out why this is happening. Any thoughts? Thanks in advance!
 

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You are trying to connect a rad tail to a rad tail extension. Totally wrong.
If you change the radiator valve to one with a 15 mm compression fitting ( rather than 3/4 inch union) you might have a chance ,but the rad tail extension may be too short to reach ,and you may need a longer one.
 
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Hello Terry and thanks for your answer.

Could you help me understand it a bit more? So to mind, I bought that lockshield valve


Isn't it what goes on the opposite side of a TRV valve?

And because the radiator is
narrower than the old one, I bought this rad tail valve extension


I thought that the rad tail extension is the same as a rad tail valve but just longer..

So, the lockshield valve is 1/2", the backnut is 1/2" so I thought they will fit.

P.S. On the other side, where I used the same rad tail extension, the backnut went in smoothly.

Please help me understand where I am wrong mate...
 
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Fair enough.. So the valve is 3/4" even though in Screwfix's website it says 1/2"... Can't understand how you can miss the description on something simple like this one... Anyway, thanks for your help.
 
The part that screws into the rad (the tail) is ¹/² inch thread. But the union that connects the valve body to the tail is 3/4 inch union.
Your TRV had a ¹/² inch tail ,but the tail connects to the TRV body with a 15mm compression fitting.
If you had a lockshield valve with a 15 mm compression fitting to its tail ,that would connect to your tail extension. ( If its long enough to reach)
 
A bit of a bodge, but given where you've got to, you could use a female iron fitting between the locksheild valve and the rad tail. It will look like a diy fudge but the connections will match up and you should be able to get it to seal. You will need to cut the rad tail a bit shorter to fit.

 
Oh and don't use PTFE tape on the thread of a compression joint, it performs no function. If you have a need to use it anywhere, then it only goes around the olive.
 

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