OK,
On to the firewall.

I first measured the distance from the face of the thrust washer to the face of the motor. It is about 14.4mm. I then measured the diameter of the nose at a point 14.4mm back. it's a little tough to do that but it was about 80mm.

I then made a depron disk 80mm diameter with a 27mm cutout to fit onto the motor.

This allowed me to double check my depth and to see if I was in the right ballpark.

Next, I cut out a 80mm ring from lite ply with the 40mm bolt circle and 27mm cutout for the motor and used this to rough fit the motor. I wanted to have quite a bit of gap between the back of the spinner and the nose of the fuse so that I could incorporate a FR4 spacer in between my motor and my finished firewall. The FR4 is very hard and will allow me to either add a shim washer here or there or to sand any necessary thrust correction in the event that I don't get my motor perfectly aligned. I left a 3mm gap so that i can add my 1.75mm FR4 ring between the motor and firewall.

I then cutout my 80mm disk from 3/16" ply

I sanded it to fit, which resulted in the front of the disk being 75mm, with the bottom of the disk extending to 78mm (see John's pic in the above post) and I sanded the rear of the disk to be 79mm with the bottom extending to 80mm. There is still a bit of a gap on the bottom of the ring, but nothing too bad and the Aeropoxy will fill it.

I then marked the air holes in the firewall and drilled them out and used a dremel disk to turn them into a more triangular shape. Not as cool or precise as something that Chad or Isaac Najary do, but it should be good enough and it was easy.

I laminated the firewall and trimmed it up. Now its ready to install.

If you want to have a tighter gap, then start with about 80.5-81mm disk and sand it to fit.

This will be it for me until next weekend, I've got some family matters to attend to. My Father In Law has passed away after a long illness. He was a great guy and a tough old bird. I loved to listen to his stories from his days as a Navy pilot in WWII. I'm attaching a pic of him flying his Kingfisher over Tokyo in 1945. I was really proud to have found a picture of him in the air. What a rare find. He was so surprised that I found this picture. A good salute to LT. K.E. Mesnard USN Ret.