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Yeah, when I completely remove the valve the radiator comes on. That is when the little pin in the radiator is fully up. So to switch the radiator off, the central plunger part in the the thermostat thing pushes the radiator pin down (when you turn the thermostat valve to cold).
I am inclined to think that the metal pins on the valve adjust the height of the plunger thing in the centre, and that it's set to the maximum depth (28). But not sure.
frankenbike
absurdbird
I think the way they work is the pin is most rigid when the stuff inside it is cold, so it opens the valve when cold. Then the stuff inside heats up and the pin rises closing the valve off. You should be able to operate the valve with a stick or something that won't damage the plastic. It looks like you have the base set for maximum heat, guessing that the biggest number where that little metal tab is pointing means that. So according to my possibly flawed theory your thermostat top is not working because it's not pushing the pin in the valve far enough down.
At least try opening the valve by pushing on the centre and if that works you know you need a new thermostat top. Quite often it's a case of changing the whole component though.
Re-reading your post I can see I've got the whole thing back to front.