Brief Description: Upper Firewall Assembly and Mounting
The upper firewall was slid behind the lower firewall, then the rivet holes were match-drilled from the lower firewall through the upper firewall with a #30 drill. The upper firewall was removed and deburred, then the two engine mount holes were upsized with a step drill to 15/32” to eliminate interference with the 3/8” engine mount bolts.
The upper firewall was clecoed in place and a couple of angle aluminum pieces were clecoed to the two firewall components to keep them aligned with each other. With the upper firewall constrained in position, the rear flanges were adjusted to be parallel with the top deck skin, to be installed later. It is important that this be done before mounting the forward flange, because it affects the offset between the two flanges.
Since the forward facing firewall flange is offset 1/8” below the rearward flange to provide for the thickness of the engine cowling, the process of mounting the flange strip is more complicated than for the instrument panel. A 1.5” wide strip of 0.025” thick aluminum was clamped to the rear facing flange of the upper firewall, and extended over the forward flange to serve as a reference surface for locating the forward flange on the firewall. Three pieces of 1/8” thick by ½” wide by ~3/4” long aluminum spacers were cut to set the forward flange with the proper offset. These were clamped between reference surface and the forward flange while the match drilling of the flange to the upper firewall was completed. A spacer was located adjacent to the tab being drilled and at the tab on each side to keep the flange parallel to the reference surface. After each hole was drilled and clecoed, the spacers and clamps were moved for the next hole. This process worked well.
Rear Firewall Flanges Being Aligned with the Instrument Panel
Spacers and Clamps Aligning the Flange for Drilling
