Assuming you want a valve that can spin forever, but is still a "wheel" - Your best bet is making a custom entity, and abusing floating point a little.
Firstly, you are going to want to make a copy of the ".ent" file of the valve/wheel entity you want to be able to spin forever. I'll assume for the moment it is "valve_iron.ent".
You will then want to call your copy something like "valve_iron_infinite.ent", and place this in your custom stories entities folder (or wherever you are storing entities for your CS/FC).
The next step is to open up the "valve_iron_infinite" entity file in notepad (or your text / xml editor of choice) - we must edit the xml data in a text editor as the model editor
doesn't permit us to do what we are about to do. You then want to find the lines that say:
<Var Name="MinLimit" Value="-180" />
<Var Name="MaxLimit" Value="180" />
If you are using a different entity the values might be different. We now simply change this to:
<Var Name="MinLimit" Value="-INF" />
<Var Name="MaxLimit" Value="INF" />
And save the entity file. You can then place it in your map and it should spin forever (or a very very long time). I can't say for certain that this will be side-effect free, but it seemed to work OK for me when I was testing it.