I am helping a friend restore interior on his Arrow IV. Underneath the rear seats, there is a plywood board where the rear seats are fastened. We put soundproofing under the carpet there and need longer screws to fasten the plywood to the airframe. Fasteners appear to be cadmium plated flat head # 8 machine screw 1 in long with 1/2 in shank. Are these structural screws (MS or AN) or can we use grade 5 stainless steel? The parts manual did not show the fasteners and local A&P was not helpful.
