This is slightly difficult to explain. I've just bought a Honeywell TRV head (TRH4M30WBG) to fit to a valve that previously had another make of head on it. The thread (30mm) is correct but the central nut on the valve is smaller than the internal TRV body is designed for, so the body tends to turn when the head is adjusted. Is there an adapter available?
Not off the shelf. Whether you could make something out of shim, I wouldn't like to say. But the odds are that the axial positioning will also be wrong. You *might* get away with it. How much is your time worth?
If the original valve is old, and/or not Drayton or similar quality I would not spend long on it.
I think thee are 28-30mm adapters from Myson, and IIRC some old honeywells may even have been rebadged mysons ...
e.g.
at that price I'd be looking elsewhere, but you get the picture.
The thread is the right size, the problem is that the body turns on the valve because the nut is too small to engage with the inner part of the body.
It's a new Pegler or Honeywell (I think) valve. The house has Evohome HR92s on every radiator and towel rail but I decided that the airing cupboard would be better with a regular "old fashioned" TRV and didn't think to check what stops a TRV body from turning. I think the axial stroke is OK so, if necessary, I'll have to make a widget.
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