Yeah I regularly assemble planes like this.

I do similiar to PMW...

However here is what I do.

- I first determine the engine to firewall distance and temporarily place the engine on the mounts using clamps.

- I then check to make sure that the engine is perfectly square with the mount using T squares, and at the proper distance.

- I drill out the engine mount, engine mounting holes as needed then affix the engine to the mount.

- I then place the engine and mount on the firewall, with the fuselage of the plane pointed straight up

This is easiest to do BEFORE you put the plane together.

- I then sight straight down the engine shaft line to align the shaft ( where it hits the prop back plate ) to the scribed lines on the firewall, if they are present.



- If no lines are present I will start on the cutouts for the cowl, so I can get the cowl over the engine, minus the muffler and/or carb if needed.

- I measure and mark the center of the firewall and draw the intersecting lines.

- I will then sight down the engine shaft and make sure that shaft ( where it hits the prop back plate ) aligns to my lines. This will properly offset the base of the engine.

- I'll apply a drop of CA or use a clamp to hold the engine & mount there, until I have a chance to mark the desired locations for the motor mount holes using a GP Dead Center Hole locator.

- Once done I remove the engine & mount, drill the holes, harden them with CA, and install the blind nuts using any of the standard blind nut mounting tricks...


It may take a little longer than a pre-done firewall, but you have far more flexibility in engine mounting if you can do it yourself.

e.g. I can do 45 degree head down ( or up ) mounts this way, which hide mufflers, etc.