ORIGINAL: whstlngdeath
What airplane is this, and is it designed for a large twin engine? By re-fabricating the firewall, it will need to be heavily reinforced and pinned as it was previously. The GT-80 is a heavy engine and the airframe has to be able to handle it. When moving the firewall back, you have to match the original thrust angles, (if any), of the original installation. If the original firewall had some right thrust built in, then the engine should be mounted slightly to the right of centerline, (as viewed from the front), to allow the crankshaft to be centered to the cowl. If the original firewall had thrust centerlines on it, then those are a good starting point. Also, the building instructions should get into mounting of the engine.
Jesse
Hey Jesse, thanks for the response, all engineering for the mounting, strength issues, that is all on me, I get that part, and am not concerned. I checked and made appropriate adjustments as I tacked the new firewall in place, it is the same angles all the way around that the previous one was.
To be more specific, maybe I need to go to the scratch builders forum if there is one. maybe all arfs have those scribed lines on them you refer to, this one did, but I guess my question if for the guys that don't have the scribed lines, what method do they use. In my mind, it might be a spinner cone with a laser pointed backwards centered in the hole of the cowl, and then mark the center of the firewall. before I go engineer a solution, I was hoping someone here might say "do this" and it would work.