On my H-9 P-47, the firewall is/was very thin. In fact, it was only doubled where the original mount holes were. (the rest was cut away to lighten the structre) Since I was not going to use a Saito 4-stroke, but am using an OS .91 Surpass which is physically larger, and since I did not want to have to, "Butcher" the cowl, I laminated in a piece of 3/16 real aircraft ply. I installed this using the tried and proven method of drilling both firewall parts, and epoxying the sandwich, and epoxying dowells through the drilled holes. Works (and has worked well for years) The engine is now truly inverted, and the cowl needed no additional hole for the rocker box to protrude through. I suppose you are correct, in that now my firewall is truly 3/8" thick. Not having any experience with ARF's other than this one, I suppose that what you have said is true. I do know that I would never trust 3/16 alone to support a high output engine. I was under the impression that he was adding to his existing firewall, and not using it as a stand alone part. Sorry about the misunderstanding.