Question: How do I position my engine on the engine mounts and on the firewall when I intend on installing the engine at an angle or inverted rather than upright?

Short Answer: Regardless of orientation, the center line of the crankshaft must be positioned over the intersection of the vertical and horizontal thrust lines for your particular model. There are a few exceptions as described below.

Better Answer: Some models pretty much force you to install your engine in an upright orientation. Other models, particularly once you move up from basic trainers, offer you a variety of engine orientation options. Rotated 90 degrees is one such choice that often works well with Pitts style mufflers...  inverted (sometimes called 180 degree rotation) is another choice. Generally these non-upright orientations, hide more of the engine and muffler and result in a more realistic scale like appearance to the model.

The choice of what angle to use is pretty much up to you, limited by the physical size of the engine, muffler, cowl etc. Some engines for example can be oriented at 90 or 135 degrees but run into a physical problem at other angles... they hit something, the muffler won't clear the fuselage, the needle valve is at an awkward location etc. When selecting an angle to mount your engine, you have to balance off appearance, convenience, practicality and most of all make sure it can physically fit. Don't forget to test your planned installation angle with the muffler and carb installed. Also check out your engine manual for any specific guidance from the engine manufacturer. Some manufacturers don't recommend an exactly inverted 180 degree installation. We have found this to be good advice in many cases... 180 degree inversions have a tendency to smother the glow plug in oil and residue if run at low RPM for too long.

Once you've decided on the installation orientation that you want, the actual position of the engine on the firewall is no different with a rotated engine than it is with an upright engine. The engine thrust is always centered around the crankshaft and regardless of orientation, the center line of the crankshaft must be positioned over the intersection of the vertical and horizontal thrust lines for your particular model. The vertical thrust line lies up and down through the middle of the firewall. The horizontal thrust line is often drawn on the firewall or is to be draw on the firewall by you after referencing location information in the model instruction manual.  

There are some exceptions but the exceptions are few and far between... if the mounting face of the engine mounting lugs is offset from the center line of the crankshaft you have to adjust accordingly. This is rare, we have not seen this in standard two stroke engines. Engine manufacturers avoid this if at all possible. If you are working with a large gas engine adapted from another industry or a multi piston engine, examine the mounting system carefully and make sure you understand the relation between the engine mounting lugs and the center line of the crankshaft.