What size is this rad/valve connection? picture included

[Last Plumber]

Maybe that valve has the large flange tail?
Was more common on the older TRVs and on the valves with option for copper or iron pipe, which that is.
The photo is difficult to judge, but the more I look at it, the more I am sure that tail nut is the large type.

I agree. It is the older type TRV with a 1/2" female thread on the bottom that you could use with the male nut and a longer than standard olive provided or any other male BSP 1/2" thread.

 

[Best]

I agree. It is the older type TRV with a 1/2" female thread on the bottom that you could use with the male nut and a longer than standard olive provided or any other male BSP 1/2" thread.

So do you think the tail is the large nut type?
I carry the large tails with a female fitting on them for draining from radiator valves or blanking off radiator valves that are the larger type

 

[Last Plumber]

So do you think the tail is the large nut type?
I carry the large tails with a female fitting on them for draining from radiator valves or blanking off radiator valves that are the larger type

Yes. I think the nut is the older thick walled one. The tails as I remember had threads that were cut across if you know what I mean. I used to look at them and wonder why. Do you know what I mean or will I need to find one and post a pic?

 

[Best]

Yes. I think the nut is the older thick walled one. The tails as I remember had threads that were cut across if you know what I mean. I used to look at them and wonder why. Do you know what I mean or will I need to find one and post a pic?

Do you mean the fine cuts across the threads that I think were done for to allow the tape, or thread seal to grip and stay in place?
I think Westherm did similar valve.
Also Tower might have been similar

 

[StuartP]

So...are we any closer to getting a definitive size for this union. I'm leaning to it being 3/4 bsp. The trv is deffo 1/2 and the union is deffo bigger than 1/2 as I've tried a 1/2 nut against it and it's way smaller. I can't see it being metric size, what with the trv being imperial. Anyone agreeing with me on this??

 

[Best]

So...are we any closer to getting a definitive size for this union. I'm leaning to it being 3/4 bsp. The trv is deffo 1/2 and the union is deffo bigger than 1/2 as I've tried a 1/2 nut against it and it's way smaller. I can't see it being metric size, what with the trv being imperial. Anyone agreeing with me on this??

That's what I am thinking. Try a washing machine hose on the valve to see it same threads.
One way to seal the valve is to screw the tail out of the radiator and seal the 1/2" end with blanking plug and then tighten it onto the valve. Have to redo the 1/2" joint with ptfe for into rad later though

 

[Last Plumber]

So...are we any closer to getting a definitive size for this union. I'm leaning to it being 3/4 bsp. The trv is deffo 1/2 and the union is deffo bigger than 1/2 as I've tried a 1/2 nut against it and it's way smaller. I can't see it being metric size, what with the trv being imperial. Anyone agreeing with me on this??

It's not 3/4".
If you are taking the rad off for decor, do what I said with the 5p

 

[StuartP]

It's not 3/4".
If you are taking the rad off for decor, do what I said with the 5p

I get what you're saying with the 5p and tap washer but the nut does not slide of the rad tail, I know this because I had the rad off to remove the wallpaper. This was only a few minutes so no need to blank the valve, I have plasterer coming Monday so will need this rad of for a good few days so want to blank off so can run the CH

 

[Last Plumber]

That's what I am thinking. Try a washing machine hose on the valve to see it same threads.
One way to seal the valve is to screw the tail out of the radiator and seal the 1/2" end with blanking plug and then tighten it onto the valve. Have to redo the 1/2" joint with ptfe for into rad later though

I have been thinking !!!!
Do you mean the type that was a proper union with a half inch tail but 3/4" nut?
I think you may well be right, I had forgotten those.

 

[Last Plumber]

I get what you're saying with the 5p and tap washer but the nut does not slide of the rad tail, I know this because I had the rad off to remove the wallpaper. This was only a few minutes so no need to blank the valve, I have plasterer coming Monday so will need this rad of for a good few days so want to blank off so can run the CH

I think Best may be right with this. There was a 3/4" nut on a half inch tail but it must be 30 years ago.
If it is that then the photo is very deceptive indeed.

You will need a 3/4" blank or washing machine blank from somewhere like wicks.
Failing that, you would need to get the tail out and take the nut off the narrow end and use a piece of lead or a 10p and 3/4" washer.

Whatever you do you need to avoid damaging the face.

I'm starting to think new rad valve would be easier.