Photos of damage to J3 Cub propeller after replacement of old engine
mount with new factory engine mount. Cowl clips rub against back of
propeller, wearing a groove the size of a nickel and a few centimeters
deep in the metal prop.

Worth checking your installation for similar problem. Note the cowl
clips lie flat against the cowl during preflight and appear to rotate
90 degrees to strike prop only when engine is running.

Photos show clips in rotated position.

a href="http://www.geocities.com/jruh/"link to photos/a