I decided to post a new thread as I posted this in an ongoing, and really this applies more to general than to a specific model.

Good afternoon fellas. About 9 months ago, a friend of mine who owed me some cash found this deal and went ahead and bought the airplane for me. I have not built or flown a propeller driven RC airplane in many years, as I have been focusing on Turbine Jets and built probably more than 20 of them over the past 6 or 7 years. When I was into props, I was into much smaller airplanes than those of this size.

Anyway, my question for you group of knowledgeable guys is that another friend of mine who had a NIB Zenoah GT80 twin sitting on the shelf in his garage has given me a long term loan on the engine (read is I legally stole it ) ( a term we use between the two of us taking things from each other) and the engine is too deep to mount to the existing fire wall, so I took it out and have tacked in place a new firewall the appropriate dimension back (1.3125 inches). Is there a tool beyond eyeballing it for centering the engine so that the prop shaft comes out center of the cowl?

I have never done this before, so please don't assume I have any knowledge of what to do, but for the purposes of the discussion you can assume I am familiar with most building techniques and tools of the trade, on the jet side of things, I mold and scratch build jets.

Pics of this airplane and situation I am in, and pics of where I primarily spend my hobby time, if you find that sort of thing interesting:

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.

Yes
If the firewall is canted to give offset thrust, you can use the following to figure out where to locate the motor mount.

http://www.lcrcc.net/offset_calc.htm

All that takes is a lot of math and measuring.

Of course, the real problem is sorting where the centerline of the engine's prop shaft (which is what located the spinner backplate) is in relationship to the firewall. And that'll take some guesswork now that the original firewall is gone.

Since the cowl has to be mounted to discover where the backplate is going to be in relationship to the new firewall's vertical and horizontal axiis, and you can't really mount the engine if you don't know where it should be for the cowling mounting, you're in a bind many of us find ourselves.

I get another modeler to hold the fuselage pointed up. Then place the mount with the engine bolted on roughly where it'll be when the cowl is positioned properly. It'll sit securely depending on the engine mount. Then place the cowl over the engine that's just sitting there and start shifting both the engine and cowl until the cowl looks right with the rest of the fuselage. Gingerly remove the cowl and mark the mount. Once the mount is attached with the engine on it, the little bit that the mount is off (if any) won't matter much at all.

Yes - it's a little tricky.
Does there need to be any cutouts made in the cowl for such things as cylinders? - if so then this is what I do.

I make a "dummy engine" - this consists of a flat ply plate with small holes (about 1/16" - just enough to get a pencil through them) in the correct location for the engine mounting holes. - in the crankshaft location (usually in the centre of the hole pattern but not always) I glue a large piece of round wood (a cut off piece of broom handle works well). That wood is longer than the engine itself but it usually has a mark on it at the length of the engine (from the back of the mounting holeds to the prop hub). I also make a cone that fits fairly neatly over the "centre pole".

From there it is easy - Put the dummy engine on the firewall, Install the cowl in the preferred position, place the cone over the dowell so that the dowell is centred iin the outlet of the cowl (I got that idea from an automotive clutch aligning tool), roll the plate around so that the holes are horizontally/verticall aligned on the firewall and use a pencil to mark the bolt hole positions (or you can just have the centre line positions marked on the plate of the tool).

You can then also remove the cone and use a rule to measure from the front of the cowl to the mark where the engine length is made and there (plus a few mm) you have the dimension of any standoffs you might need.

Unfortunately, I am not near my shop at the moment or I would photograph one - I hope the description above makes sense.