How deep does the 51% rule apply?
The issue is sub-assemblies? When is a buildable assembly a component? Is there a bright line the FAA uses? We make brake systems. We build ribs. We install radios and instruments. Yet we're not expected to raise our own trees, extrude aluminum, or make semiconductors. We are expected to use those materials intelligently.
Instrumentation and avionics traditionally have been seen as components. But they don't need to be. If I wanted to provide a kit that enabled builders to make an (experimental) aneroid altimeter, would the 51% rule require the builder to construct the bellows? Would the labor involved add to or detract from the builder's "larger" 51% responsibility for the whole aircraft?
How about software? When the rule was created software didn't really exist, at least not in its current form. If I wanted to make software available to builder-pilots for their experimental avionics, would they need to write 51% of the code?