The firewall bulkhead for wooden aircraft is usually plywood and must be protected by a metal firewall. It must also be inaulated from heat passing thru the metal firewall. Tony Bingelis in "Engine Installation Methods" suggests using a layer of fiberfrax ceramic insulation between the plywood bulkhead and the metal firewall, but does not indicate the thickness of fiberfrax to be used. Dave Prizio in sept 2012 Kitplanes suggests placing the metal firewall directly against the plywood bulkhead and placing 4 layers of 1/8 inch fiberfrax in front of the metal firewall and sandwiching that with a sheet of aluminum. I would like to know what other builders are doing in this regard and if there is a current standard to be met in firewall construction for wood aircraft. Randy McCoy