Does Blender allow edges to maintain edge length during scaling? Note that I am not referring to axis lock, but to the ability of marking specific edges to be immune to resizing, but still movable during scaling.
The intended usage is so that a (2D) ring can be resized to a new diameter without changing its thickness. Thus fixing scale problems when modelling with varied sizes cylindrical objects. :eyebrowlift2:
If ring is on xy plane, thickness is on z, then s followed by Shift-Z allows for resizing diameter only.
That relates to your last sentence, still i don’t quite get the first one…
But that does not retain thickness 
(We are talking 2 bridged circles that can have there diameter changed, but distance between the edge loops stay fixed.)
Edit: I am not refering to Z thickness, by “(2D) ring” I meant the model is flat on a plane of 2 axis.
Thinking (understanding) sometimes is a hard business :). Yep, that makes a difference, Radii would scale proportionally. Haven’t seen function that does that…However if yo turn on edge length and check initial distance, you could scale inner circle up to that number after rescaling all, better yet, draw diametrical edge from outer ring that long and then snap scale inner one.
Thanks for the reply, I think a method of length restriction would be a usfull tool, just seems like a more efficient way than than adding a guide Obj. It would be like using weights when using subsurf. Whenever I get around to Python I will try and make a script to do this, when I get around to it…
I look at the problem in terms of pipe wall thickness. Where pipe diameter changes but not the wall thickness. That points to Solidify Modifier. It sets thickness independent of pipe diameter. It works on 2D circle as well!
Scaling should be done in Edit mode however. In object mode, thickness scales with circle but the thickness value does not change. It must be a bug in the system!?