The firewall recess F-601K-1 is stainless steel and is riveted to the firewall with AN426 rivets. The recess itself has 6 AN426AD3-3 rivets that hold it together. I clecoed and match drilled the 6 holes to #40 and deburred and dimpled the holes. I used my pneumatic squeezer to set the 6 rivets. I had made the bottom stiffener F-601E-1 earlier, but the holes needed to be machine countersunk for AN426AD3-4 rivets. I accomplished the countersinking and clecoed the stiffener and firewall recess to the firewall and checked the fit. I got the rivets and tools ready and removed the firewall recess and applied a bead of CS1900 firewall sealant to the flange of the recess and clecoed it back to the firewall. I used my rivet gun and and bucking bar and back riveted the recess to the firewall. Last, I applied CS1900 sealant to the seams and openings in the recess.
