Gino,
On my Phantom the first section of thrust tube is 11 " long and starts at 5 1/4 " diameter at the fan and tapers to 5". The second section is 13 1/2" long and tapers from 5" to 4" .My tail cones are about 3" long and will taper down to 3 1/2", The hump for the cylinder head is 7/8" taller. In the good ole days, you could buy the engine cover separate and glue it onto your homemade tube. I thought I had one of those but I'm unable to locate it. I'm thinking of scratch building a F-20 for Dynamax/OS.91 and will have to make my own inlets and tailpipe for it.
I have included some pics of the inlet and tailpipe for a JMP Starfire II I built back in the '90s. The thrust tube on it goes from 5 1/4" at the fan to 3 3/8" at the exit.
I hope this information helps.
