Oka, finally time to do the last assembly on the firewall. Here's a hole, prepared for the installation of a bucked rivet. That a 100 degree countersink. Aircraft guys have extra special tools.
Here's what the hole looks like with a rivet dropped into it, and another rivet beside it to show you what it looks like before it gets bucked.
And here are the tools - a pneumatic rivet gun and a block of steel to act as the "buck" - the anvil you hold on the back side. You can see the end of the rivet sticking up into the countersunk hole; the countersink actually acts as the form for this end of the rivet. You just hold the buck against the other side, and use the pneumatic hammer to smash this end until it's flush.
Like so. Yeah, it's not perfect, and there are some apprentice marks on it, but this is the back side. I did it that way on purpose.
This is why I am making the whole thing removable - so I can service the fuel tank down the road, so to speak.
Here's the firewall pulled out again for today's work - making those little notches at the corners, to clear some weld bead on the frame, and then to mark and drill for the row of screws which will go along the top edge to secure it to the frame.