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Discuss Radiator Valve Threads in the Central Heating Forum area at

  1. acsmith1986

    acsmith1986 New Member

    Hi All,

    Not a plumber but pretty handy with tools and experienced DIYer, however have come across a 'good one' that I am struggling with and wondered if someone would be able to provide some help.

    I have a 1930's house and a set of radiators with what seem to be very odd tails on them. I need to remove the rads to decorate and the valve are leaking (1st problem) which means I need to replace them with a compatible thread patterned thermostatic valve (2nd problem)

    I have been to a few plumbing centers and everyone I have spoken to says radiator tails are only one of two sized 1/2" or 3/4" so they have sold me the cap end nuts to stop the leaking while decorating, so I can replace the valves when I find the correct size.

    I have taken a few measurements and can uploads some pictures if needs be.

    Thread width = 21.5mm
    Thread spacing (peak to peak) = 1mm
    Parallel Thread

    If anyone could help me with what this may be it would be greatly appreciated!
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Post some pics of the valves please
  3. acsmith1986

    acsmith1986 New Member

    excuse the mess on the walls, the rad seemed empty, then it wasn't!
    20180611_205611.jpg 20180611_205616.jpg 20180611_205623.jpg 20180611_205629.jpg
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    1/2 into the rad 15mm copper
    • Agree Agree x 1
  5. acsmith1986

    acsmith1986 New Member

    I have a 1/2" end cap and it doesn't fit, sorry I thought I mentioned this above but i missed it.
  6. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Which bit are you trying to put the 1/2” into? Both connectors on the valve are 15mm the hole in the rad where the tail goes in is 1/2”.
  7. acsmith1986

    acsmith1986 New Member

    So I am trying the cap the valve off because it is leaking, the valve has the male thread sizes as mentioned above the rad tail has the female 'nut' I am trying to put the 1/2" cap / end stop on the valves male thread.

    Hopefully you can understand what I mean.
  8. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    You won't be able to cap the valve off as it's old thread

    But remove the tail from the rad and put the cap end on the threaded end
    • Agree Agree x 1
  9. acsmith1986

    acsmith1986 New Member

    So you are saying the radiator tail thread into the rad should be 1/2"?

    Would new radiator tails be 1/2 thus enabling me to replace tails with new ones and matching threaded thermostatic valves?
    • Like Like x 1
  10. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Correct if the trv is new eg last 10 years it should fit

    The one in the pic isn't a trv it's a lock shield valve
    • Like Like x 1
  11. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Sorry for the confusing post about. Just read it back and that isn’t what I wanted to put but as normal I got distracted. Sorry.
  12. acsmith1986

    acsmith1986 New Member

    thanks for the input, so to summarize, the lock sheild valves have an old thread that obviously matched the rad tails. but the rad tail thread should be 1/2"? if the outer thread is an old thread why would the thread that goes into the rad not be something other than 1/2"?
    But if it is indeed 1/2" I can buy new tails for the rads with 1/2" BSP connections which will fit a new TRV with a 15mm to 1/2" connection?
  13. snowhead

    snowhead Well-Known Member

    The thread in the radiator WILL be 1/2" bsp and will take any 1/2"current valve tail.
    If you want to cap the valve, loosen the tail and use the cap on the radiator end of the tail.

    The thread on the valve compression nut(s) at 1mm pitch would appear to be manufacturer specific as Bsp 1/2" is approx 1.8 mm pitch.
    1/2" compression nuts are almost always 1/2" bsp.
    The thread on the nut on the pipe into / out of the valve will also be most likely to be non standard so you'll have to remove the nut and olive and use the ones supplied with the new valve. (this wood be good practice anyway)
    • Informative Informative x 2
  14. acsmith1986

    acsmith1986 New Member

    Thanks for the help! will give getting the tails out a go at the weekend and see how it goes, fingers crossed!
  15. oz-plumber

    oz-plumber Plumber

    I would just about put my house on it that the thread size on the valve to connect the tail would be 3/4" BSP.
    If you undo the large nut between the valve and the radistor - that thread should be 3/4" BSP
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