Cracking header pipes

A returning problem is cracking of the aluminium Hatori header pipe (#334 for shuttle). The header pipe is between the engine and the tuned pipe:

Hatori 334 Header pipe.

My first header pipe cracked after about 80 flights. It cracked at the inside of the first bend after the engine:

First pipe cracked after about 80 flights.

My second pipe cracked at the outside of that same first bend after the engine. A bolt fastening it vibrated loose, after some oil from the engine had crept between the nuts. Although I noticed that the bolt was loose during the standard maintenance and after only about 4 flights on that day, the pipe was already cracked.
My third pipe was fastened even better, to be sure that the pipe would not vibrate loose anymore, and I now always check for oil on the bolts. It lasted about 100 flights, but again a crack appeared at exactly the same place as the second pipe.

Second pipe, 30 flights.

Third pipe, 100 flights.

I don't think that a life time of 100 flights is normal. I tried to claim guarantee with the first pipe, but it was rejected because I could not prove that my heli did not have a vibration problem. I don't think I have a vibration problem, as the fuel is reasonable stable and the only part of the heli having vibration problems is the header pipe.

To avoid cracking, I now have split a new pipe, and replaced a short part with silicon tube. This configuration has hold quite some flights, but since march 2002 I fly with an OS50 using a standard muffler. The hatori 541 header I have flown with shortly can't be split: there is no flat end on the header between the two bends, and the silicone tubing pushes itself off the header...

Pipe split in two.

Middle part replaced with silicon tube.