I have a 4 star 40 that had a kb 45 on it. I flew it for the first time the other day, and after 5 minutes of flying, the engine just quit. I did a good dead stick landing and decided to replace the kb engine with an OS engine. The kb engine has a motor mount on the backplate that screws onto the firewall. I have a nexstar engine mount that I would like to use, so my plan is to cut a piece of plywood the size of the firewall, mount blind nuts in the cut piece that fits the Nexstar mount and screw and glue the plywood into the firewall. I drilled the additional firewall to match the existing screw holes in the existing firewall. The top two screws fit nicely in the plywood and firewall, but the bottom two screws will interfere with the new motor mount. Now I plan to run wood screws through the plywood into the firewall without using the existing holes. Is the existing firewall about 1/4th of an inch thick. Could I use 3/8 of an inch wood screws and not screw into the fuel tank? Another question is can I just center the new motor mount on the new firewall and be ok with the settings of the engine? What pitfalls might I run into doing this way? I suppose the proper thing would be to remove the fuel tank and drill in new blind nuts, but I would like to leave the fuel tank alone if I can.

Thanks

Carroll

if i understand you right you have existing holes and two of them will work but the other two dont?if that is the case just use wooden dowel stock the size of the two old blind nut holes glue it in and redrill the firewall for the new motor mount and put in the blind nuts.

take horace's advice, don't add the extra weight of the extra firewall. if none of the old screws are in the way you don't even have to fill them just drill it for the new mount and install new blind nuts or pop the existing ones out and reuse them.

kc