RE: 4*120 Build Thread
The firewall is plenty strong. Just put tri-stock behind AND in front of it. Then pin it. I like to use bamboo scewers and soak them with thin CA. Rock hard and cheap.
The engine is going to shake like nuts compared to a 4-stroke glow engine. But the airframe will handle it. No problem. This fuselage is solid lite-ply. It's built like a Russian outhouse. Just use plenty of Medium CA on the fuselage and you'll be fine. I have mine glued solidly and there's little fillets of medium CA everywhere I could get them.
My Ryobi engines that I've converted in the past usually will swing an 18-6 just under 7000RPM.
It's generally recommended that you don't exceed about 8000RPM on a Ryobi. If you've ever had one apart--you'll know what I mean. If you haven't had one apart--trust me. The rod is just 2 simple pieces of metal thats stamped together. The crank counterweight is just stamped pieces of metal too.
The engines have plenty of torque, but they are NOT a high RPM engine. I would prop it for 7000RPM on the bench and let it unload in the air. That should keep it under 8000RPM.