Scott, et al, ..........the mounting screws for the cowl attachment will be going around the edge of the firewall/bulkhead. You can't get in behind there to put in backing plates or even washers for that matter. However, It is a good idea though to use backing plates, with blind nuts, for mounting anything to the firewall. Then you can detach things without someone standing on their head behind the firewall holding the mounting nut. We have backing plates with nutplates for everything, even the motor mount; four backing plates with 4 nutplates each.......16 of them little boogers. But one person can pull the motor mount......
KRJerry Jerry Mahurin Lugoff, SC e-mail: krje...@bellsouth.net Website: http://KR-Builder.org . However, you could back-up the plywood by placing an angle or a flat doubler behind the plywood to make a sandwich. That way you don't have the end of the rivet unsupported. I would be hesitant to use the pop-rivet back-up washers however in this critical area. Since this is a Firewall, you'll need to make sure that every penetration is sealed. .
