Today I added more of those firewall parts that I've been looking at on other peoples web pages for a couple of years now. Hard to believe this is MY firewall. The center stiffener was fairly easy to attach. I was very careful to get it centered, and used c-clamps to hold it in place while I back-drilled the holes. The side gussets were a little more interesting. The gussets have to fit up inside the horizontal stiffener, but here's a fairly large radius to the bend inside it so it takes a little finesse to make the corner relief holes larger and tweak the flanges down to fit flat against the stiffener. Just go slowly and it all fits together perfectly. I was amazed at how my scratch-made parts fit perfectly - not one millimeter to spare. Being careful and taking extra time getting things right while making parts pays off in the end! The plans are a little sparse on exactly where to place rivets for the gussets. I'll go through the plans tonight and see what I can come up with. It'll all fall into place.
