The motor now have 316 minutes of running. That problem start to hapen when he have 200 Min. Till there it has not this problem.
By the Minute 75th of running I've have a small crash with other plane where this motor was mounted. The crash was on "soft sand". I've take the motor out and cleaned it, and mounted in the actual plane.
Is there any possibility the "Cranckshaft" be slightly bended, and cause some vibration on the backplate to unscrew the "retainer screw" ?