Thread Tap for Cast Iron Radiator

[Gammy leg]

Hi,
I had a cast iron radiator delivered but the thread in the bottom of the rad that the TRV screws into had been damaged about 1/2 way along. I'm hoping to tap a new thread but need to know what size tap to get.
The only info that I have is that the valve supplied is 1/2" x 15mm.
Can someone confirm what size Tap I need to buy?

(btw, the supplier has told me to scrap the damaged rad and they'll send another to replace it. Hoping to fix the "scrap" one to use in another room )

Thanks

 

[CountryFan]

Tap? Do you mean the tail? Usually 1/2" if yes

 

[avonmouth]

By tap he means re-thread as in tap & dye set.
Half inch BSP is what you need.

 

[Gammy leg]

Excellent. Thanks guys. 1/2" BSP it is.

 

[Exedon]

As someone that's been around longer than most I've had a 1/2 in tap amongst my gear for over 50 years.
Very common for rads to need re tapping before compact rads came in.
I believe the tap you need is called a 'second pass' .