[1QuickS]

Baking required for high strength steel only, mild steel is not susceptible to hydrogen embrittlement; this would include Grade 5 hardware (Metric 8.8) and lower yield strength steel. The fuel rail components are all low yield steel but springs are definitely high yield.

You can plug the passageways but note that the OEM parts are plated including the threaded ends. Plating will not occur within the part but the electrolyte used in the plating bath will. You can make rubber plugs to seal off the passageways but the preparatory processes to plating (acid bath stripping & pressure washing) may dislodge them.