RE: WACO YMF
Jim (SCM),
The wooden strips (either varnished or painted) are part of the fuel tank support system. The lower center wing surface is plywood (I belive 1/4" ) and the upper center wing surface is the fuel tank top itself on the YMF Classic NC 14081. The center section is covered and painted and then the strips are screwed in place from the inside and/or also bolted. The tank is then installed and covering is completed if needed. Original YMFs had the tank buried under the covering which then had to be removed and replaced to service the tank. You are correct in your observations that some Wacos had more than two wood strips.
P1 YMF Classic, NC 14081. 2 wooden tank support strips. Notice the relief cuts in the plywood tank compartment floor to allow for the tank fittings and fuel guage
P2 HARMs UBF-2, NC 2091K. Note the 5 wooden strips with no visible screws except for a bolt at each end of the center strip. Note the rib extensions at the rear of the tank compartment.
P3 Cabin Wacos have the wood strips too. Bill Harter’s YKS-7 NC 17474, Note the 3 strips under the tank. That is Bill and his wife in front and his dad in the rear seat.
P4 Tank compartment on the upper wing of a YKS under restoration. Note the 3 lines of screws for the wood strips and rib extensions on the aft side.
P5 Upper Tank fittings on UBF-2 NC 12002 with fabric covered tank. Note neoprene washer around fill cap neck and vents and 2 extra layers of fabric reinforcement around the vent tubes. Bolt on metal center strip passes thru the wing to the center wood strip below.
P6 Top of tank on YMF Classic NC 14081. The wing surface is the tank upper section. Similar fittings.