The Firewall construction was not unlike the other bulkheads I had recently completed: the core was cut to match a template then fiberglassed on each side. The main difference here was that I used a .25″ thick birch plywood sheet as the core instead of high-density foam sheet. This made the part substantially heavier than the other bulkheads, but since this is what the engine ultimately mounts to, weight = strength = good.
None of my tools were suited to accurately cut a highly curved piece of plywood, so I borrowed a jigsaw from a friend which worked quite nicely. Plywood doesn’t sand as easily as foam (duh), so I cut the piece as close to the template line as possible to minimize the amount of edge sanding required. Glassing was as before, except these were the largest pieces of fiberglass I had used yet – roughly 3’x4′. The other significant difference between this part and previous bulkheads was the inclusion of integral hardware. (Hey, I finally got to use my cool Ingersoll-Rand pneumatic drill.) If you look closely at the photo, you’ll notice 4 silver dollar sized circles in the “corners” which are the engine mount attach points. They are actually .25″ thick aluminum plugs which were bonded into the plywood before the fiberglass was applied. Firewall screws Additionally, after the first side was glassed I bonded in 6 (.190″) countersunk screws (3 on each side) which serve as attach points for the rudder cable pulleys. (Hey, I finally got to use that 100 degree countersink I bought at Boeing Surplus.)
The hardware was bonded to the plywood using a slurry of epoxy and cotton flox (which is finely ground cotton–almost like a powder–for those of you not “in the know”). I had some doubts about the structural integrity of the screws bonded in with this mixture, and their ability to withstand a decent torque. But once it cured, I was a believer: cured flox is some tough stuff. The heads were flush on the other side, and glassed over, so unavailable for a screwdriver.
That was about it. The completed part was set aside for installation sometime in the future when it will get fire-proofing materials applied and a metal face sheet.