The amount the pins travel, and the start and end points of the travel, will vary with each make and model of valve. As to if the travel is within limits, one simply does not know.
So when the head is designed to fit many makes and model of valves, there is normally something to show if it has mated correctly. This
is from the electronic eQ-3 head
and shows if all is working as it should, the same applies with most electronic heads, but to put it simply, with mechanical heads you have to replace like for like, and when I paid in 2019 pre Brexit £15 each for the electronic heads shown, it seems daft not to use an electronic head.
There are four levels of electronic head, level 1 like the Terrier i30 there is no wireless link to any device. Next level is Bluetooth like the one shown, so they can connect to one phone, or to another TRV head, so when two radiators in the same room, they work together. The next is connected to WiFi, so it can be used with geo-fencing, and controlled with phone from a remote location, and the final stage is linked to some hub, so it can actually fire the boiler. I don't have any of the first level, but do have level 2 to 4.
Very easy to just say go the whole hog, but the ability to fire the boiler, needs to be used, where the location is unlikely to fire the boiler when not required. A radiator, against an outside wall, tends to be colder than the room in general when the heating is not running, once running no problem, it is Autumn and Spring, when the weather is cool, but the house still warm enough not to require central heating, I have to turn down the linked TRV head when Spring arrives, and then turn it back up as Autumn arrives, and the house has cooled.
But the non linked I can just leave, as even if they do open, the radiator will not warm up until the boiler runs. They have a weekly exercise where they fully open and close the valve to prevent the valve sticking. Only downside is 20 AA cells replaced once a year.
