JMCJET

You must be sure that the air can flow betwin the two pipes. The holes in the outer pipe are only needed if, for some reason, you have not enough air flowing betwin pipes. When the engine is running, the high speed of the exaust gases build depression in the exaust pipe. So, the air must fill the "space" free. If this doesn´t happen the pipe collapse. This is true for both pipes, but is more critical in the inner one. The gap betwin the two pipes in the rear end is a very smart idea because it sucks the air from the other side (turbine side). This air flow keeps the rear of the plane cold. Your problem is that the air can not go inside the gap of the two pipes and than you have depression and a collapsed pipe.

So: 8 mm gap in the rear end is OK. You must check why the air can not go inside the gap betwin the two pipes. Fix the problem and try again. If the rear end of your plane is still hot then you can drill some holes in the outer pipe to allow more air inside the gap of the two pipes.

Saudos is not my name, but our way to wish the best.

Saudos
JMC