the FIREWALL!!! the firewall is a big sheet of stainless. all the framework that makes it stiff gets match drilled, and it kind of assembles like a puzzle. the pieces all fit together just so. then you dimple the firewall so it's flush on the engine side. then rivet rivet rivet. after that, you attach the big ribs that form the tunnel, and then drill and rivet the nose gear tensioner. to do this, it really worked well to use the Cframe dimpler with the rivet gun attachment. it's basically back riveting. that's us in the last picture pretending to go flying. weee.