Alright to answer your question first, it looks like you have the Arm workplanes fixed but the insertion point for the part is between the two connections where it should be. Meaning as your connection sizes increase they are simply overlapping. You need an off set workplane not a fixed one and a calculation to determin the offset required from the damper centre.
Just a few of my rules when it comes to parametric parts and anybody feel free to correct me. Keep it simple as possible; instead of modeling your profiles off of workplanes rather extrude them along geometry. Its easier to control a single line geometry extruded profile than a workplane profile. There are just way less point to have to control as well as dimension, this also allows you to use the same profile again extruded along diff geometry. Use your contraints in these cases, they can save you alot of dimensioning.
I have attched a quick example I did based on what you attached. Once you get the hang of it its becomes very quick to create.
Ryan Smit, South Africa
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