From what I have seen from my Jet Guys who have models with pipes, there is typically about 3/4" between the tail cone and the beginning of the pipe's mouth. I have helped resolve starting problems when the tail pipe is too close to the end of the turbine's tail cone (i.e. turbine would not start w/o flames, popping and rumbling. Flames and popping/rumbling stopped after increasing distance between mouth of pipe and tail cone.) As I understand it, air needs to flow into the pipe thru the space between the cone and the mouth of the pipe. We are not just tunneling the turbine exhaust.