FUSELAGE FIREWALL
The engine I picked was the Kolm 77cc 4 Stroke with the large prop shaft extension. I did that because the cylinder head is so tall, I needed to put it as far back into the slim cowl as possible.
This choice makes it so I don't have the head hanging out on the bottom and ruining the scale look.
For future reference to new builders the distance to the FRONT of the Cowl fro the forward edge of the firewall is 185mm. If you are using a different engine that number will guide you if you will have enough room with out removing the firewall.
With the Kolm engine, that meant I had to remove 43mm and add a new firewall. The trick to making an accurate cutting line is to put the fuse rudder up and on its firewall and scribe a line all the way around. I then added two layers of 1/4 inch flexible tape to help me guide the Dremel diamond wheel as I cut the nose off.
There is a lot of discussion in some of the other build threads about how much offset needs to be built in. NONE NONE NONE! This information is right from CARF.
I built the firewall from 1/2 inch Birch plywood. I laminated the edges and inside with WEST Epoxy (slow cure) and added WEST 2 inch wide carbon fiber strips to give it extra strength. On the outside I put Finishing Epoxy to completely fuel proof it.
Here are the weights before and after.
ORIGINAL FUSELAGE 1350 grams
ORIGINAL FIBERGLASSFIREWALL REMOVED 66 grams
ORIGINAL WOOD FIREWALL SUPPLIED BY CARF 106 grams (but no glue or reinforcing tape)
NEW FUSELAGE WITH NEW FIREWALL 1620 grams.
So I have added a total of 270 grams. It would have been 170 grams with the CARF Firewall system as it is somewhat thinner.
At least the weight addition is up front.