Threads cut incorrectly on new radiator?

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Hi, I have fitted quite a few radiators over the years but have a new problem. I have bought a new column radiator - fairly cheap and no doubt imported. I've tried screwing in radiator tails dry, ie no tape - just to make sure they fit - and only get half way in before becoming really tight. I can force them further with a spanner, but on removal I can see the thread of the tails is being damaged. I've tried a few different tails and they are all the same. I am concerned that ptfe will never get a seal due to the tight metal to metal contact. Is there a way around this? Or should I return the radiator for a refund?
Thanks in advance for any help anyone can offer.
 
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Aren’t they supposed to get tight halfway in? Seems normal to me. Just tape them and screw them in - I’m sure everything will be fine.
 
What is the length of thread ,of the tail, that goes into the rad ? And what is the length of the thread in the radiator ?
 
Mottie, thanks for the reassurance, I probably will give it a go. I won't be ready to install it for a few weeks, so I just wanted to identify whether I had a problem or not. Terry, the tail has a thread approx 13mm long, and as best as I can measure the thread in the radiator is around 12mm. I thought tails generally were quite a loose fit, hence needing 10-20 wraps of ptfe to take up the gap and make a seal, however as you can probably tell I'm no expert on this!
 
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If most of the length of the tails 13 mm thread goes into the rad then it should be fine.
 
if you do recap it, cover the tap in thick grease.
It will collect the scarf so it comes out with the tap
 
If they're the older type tails (with a flange on the valve end) then long as you're getting 5 or 6 turns in it'll be fine, bit of Boss white or PTFE if you want to be sure. I got fooled the other way round (with modern tails that you put a compression fitting on)- couldn't believe how much PTFE I had to put on them before they sealed!
 
Thanks guys, I do like the idea of running a tap through it. Is this the right one? RS PRO HSS 1/2 in BSPF Straight Flute Tap Tap Set | RS Components (rs-online.com) - not sure what the F is in BSPF. Do you just turn it in with a spanner?
BSP -Fitting. With 14tpi it is the right one but very expensive just for cleaning an existing thread imho. You can use a spanner instead of a tap wrench but keep the tap true to the hole and it shouldnt need a great deal of force.
 
Thanks for these comments. I agree it's expensive - it's a 2-piece set with a "plug" and a "taper". Doesn't mean a lot to me so I'll investigate further before buying anything. 'oldbutnotdead', that has also been my previous experience - I have modern tails with compression fittings for the valve, so was expecting to use a lot of ptfe. I've just tried 10 turns of ptfe and I can screw them most of the way in - but still with that unsettling metal on metal grinding which I worry is destroying the effectiveness of the ptfe.
 
The "Plug" tap cuts a thread to the bottom of a blind hole so is of no use to you here
 

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