I'm getting close to final fitment of my converted Homelite to a scratchbuilt big foam low wing. What would be the typical thickness of a ply firewall to mount a Homelite 25cc? I've test run with a 3/4 piece of rubbish ply I had lying around but an planning to use something better. To me 3/4inch seems a bit of overkill so, what are you guys using?

Raven,

For my son's Seagull Models 60 size Edge I reinforced the stock 1/4" inch ply firewall with triangle stock. Worked fine. Being an ARF the plywood is not of the same quality you can buy here in the USA in my opinion. With that being said, the 21cc Echo conversion was fine on it. For your project I would not want to use anything less than a good aircraft grade 1/4" 5 ply plywood. For any engine that is bigger and for which I have a choice during construction or retrofitting I use 7 ply aircraft grade 3/8" plywood. Hope the pictures attached helps.

Tim

Thanks for a clear response. Sounds like I'm on track here.

Present plans are a 9mm thick 5 ply firewall (approx 1/3 inch and over your 1/4 recommendation). Local club gas experts suggest this will be sufficient. It's not 7 ply or aviation grade but far superior to most other plys I've seen (including the 3/4 inch stuff I used for my engine stand).

The firewall is supported at 4 corners by interlocking timber bearers off the firewall, which will probably have some right angle alloy added as extra insurance. Behind the firewall it is supported around all four sides with more bearers (square not triangular).