When I put together a firewall from a kit with designer recommendations for right and downthrust, I like to install the firewall angled right and down. Often this feature is built into the kit; but if not, then I build it in myself. Then there is no funny business with washers or shims. If the design is off a bit for my choice of engine, then I trim with shims and washers from there. This can only be determined through flight tests.

So, if the firewall is angled, and you center the engine mount on the face of the firewall, your prop hub some four inches out from the firewall is going to be off center from datum by some fraction of an inch too far down and too far right. To compensate, you shift the mount relative to the center of the firewall by the appropriate distance up and left. You can work out these distances with pencil, paper and protractor or drafting machine or CAD software or trig tables or trig functions on your calculator, depending on the vintage of your schooling.

If the cowl supplied with the kit assumes no offsets, then the spinner mounted to your propshaft will not line up with the ring of the cowl. Now one of two things will be awry--either the cowl will fit the fuse at an odd angle, or the spinner will interface with the cowl nosering at an odd angle. The only way to fix this problem is make a new cowl, by mounting the engine with proper offset, then fitting a plywood dummy nosering on the propshaft, then building out and shaping from firewall to nosering so that everything is harmonious. This is a lot of bother for the average sportflier, but very absorbing to the scale purist who has lots of time and patience at his disposal.