Hi Dave, I use aluminum plate to make rear mounts also. Since you mentioned mounts, I try to keep the mounting plate at 3"x4" with the firewall mounting holes the same pattern for all engines if possible. I keep all thrust lines on the mount as close as possible when the mount is bolted to the engine. I am ready to start designing a couple of airplanes for these 25cc engines. Keeping the mountings the same will make it easy to try a different engine on the same plane. There probably won't be a cowl, at least not at first. I don't think there is enough airplanes for this size (21 to 25cc) of engines. I am not talking about 3D aircraft. Your description of grinding the rear of the engine prompts this question. Did you retain the green plastic backplate cover? I prefer the Weedeaters with the 4 bolt backplate, 21 to 25 cc, and I uses spacers about 5/8" long between the "green" and the aluminum plate. I have a Brand new FL-25 I am working on, I need to make a hub and spacers for it and I will be able to test run it. I will post my results on Two 21cc engines in a another post. Thanks for confirming the metric sizes, for the Toro, they are what I found. I ordered the 8x1mm taps on the net, because I could not find them locally. Check this idea, The 8x1mm bolts for prop retention are not as common as 5/16"-24tpi SAE bolts. The 5/16" bolts are commonly used on conversions of this size so I plan on the rear of the prop hub to have the metric threads and the front of the hub to be 5/16"-24. The rear of the hub needs to be a bit less than 7/8" diameter and the front of the hub will be 1.5" diameter, so there is no chance of it getting reversed. On the Toro engines have you seen both the 2 bbl and the 1bbl carb versions. The 2 bbl version has a different cylinder head and a lot different intake porting arrangement. The exhaust porting is the same for both. I am not talking about the timing of the porting, just the phyical "tunnels" through the cylinder. Have you run your Toro yet?