Hi Florent,

There is normally a dark grey friction martial on the inside of the straps. It’s meant to be there.

Normally you'll notice the straps are a bit tight right from the point where you first install the M4 fasteners, and it should grip the turbine quite firmly when the screws are all tightened down such that the flanges are in contact. If you are concerned about it and you're in a hurry to run it, I'd say try adding a washer or two between the flanges to space them apart slightly. If you do so, please watch for any shift in the turbine's position while running... If its too loose, you’ll notice it shift forward until the glow plug contacts the strap. Otherwise, please try contacting the dealer you purchased the motor from in the off-chance that the parts were formed out of the manufacturer's spec.

Good luck,
Kelly W