Your my new Bestest Friend BB57!! you have given some very good information here to help my claim. Might have to call on you again if that's o.k.?
I have seriously looked at the 180Hp STC and have flown them, performance is definitely as expected. I would really like to do that but I am trying to stay behind the point of no return cost wise on this project and look for a Scout for my next!
Sure feel free to post questions. However, my experience is limited to the vagaries and pitfalls of reg writing.
Given how ACA jumped on the wood wing spar SB and AD, when there were to my knowledge only a couple failures (and all on aircraft with over turn or wing tip strike histories), I get the feeling their intent with 125 and Note 13 was to strongly imply that metal spars are required, so that they'd sell more metal spar wings.
That said, if it isn't expressly forbidden, then it is permissible, and any ambiguity will go against the author. The fact remains that note 13 is all about CG and gross weight limit changes. If ACA really wanted to expressly require metal spar wings for 160 hp and 180 hp installations, they easily could have made Note 14 specifically state it as a requirement. But they didn't. Most likely because there were already wood wing sparred 160 hp and 180 hp Citabrias out there in the wild being operated safely every single day. For example, I flew a 180 HP 7BGBC Citabria in 1985, almost a decade before the metal 7-1545 wings even existed.