Rad valve temporary cap size

[Welderpaul]

I need to cap off a few rad valves where the radiators have been removed. They will be off for a while whilst some tanking work is on the go so i'd like the assurance they won't start weeping, or worse.

I have these caps in my plumbing box. The 15mm cap is too small and the 22mm cap is too large. They are modern valves. which cap do i need to buy?

 

[Kayos]

3/4

https://www.screwfix.com/p/flomasta-bsp-blank-nut-3-4-x/39553?ref=SFAppShare

 

[Seadog]

It doesn't look large enough for 3/4. Maybe 1/2" BSP.

 

[Kayos]

It doesn't look large enough for 3/4. Maybe 1/2" BSP.

15mm compression blank would have fitted 1/2"

 

[hunter27]

Remove the tail from the radiator, refit it to the valve and cap the end with a 1/2" BSP cap

 

[brightspark]

if its not leaking when the radiators off why bother ./its not likely to start after ive had a few off for a month

 

[Robotstar5]

When our builder took a radiator off, he had some nifty tools he called "spoons" to block the radiator outlets from leaking.

 

[Agroshield]

Just in case it is not obvious, the thread on a 22mm compression fitting is rather an oddball, which is why it would not fit the radiator tail.

That is why you can buy 3/4" BSP to 22mm compression adaptors. If the thread was the same, the adaptor would not be needed and there would be no market for it.

The normal thread, as above, is 1/2" BSP or 3/4" BSP.

In case it is ever needed, the thing required if you take the head off a TRV is known as a (TRV) builder's cap.