This project evaluated the effect of key-lock inserts on the stiffness of bolted flange joints compared to standard threaded holes. Analytical calculations, finite element analysis (FEA), and experimental testing were used to quantify joint stiffness across multiple materials, including aluminum, steel, titanium, and Inconel. The study involved developing analytical models, creating FEA simulations of the bolt–insert–flange system, and conducting load–displacement testing using custom-machined specimens. The results were used to assess current modeling approaches and support improvements to stiffness prediction methods for bolted flange applications.