Spinner backplate rivets

Too All,

During a condition inspection for a RV-8, O-360, Blended Foil prop, approx 800 hrs on engine/prop; missing rivet heads (Manufactured head) were on the spinner backplate. They were both in the same quadrant of the plate, looked like they were simply sheared off. Backplate and spinner show no signs of vibration or cracking. No indication of unusual vibration anywhere on the airframe.

Anyone observe this?

Had 2 planes come in exhibiting this. Replaced the broken rivet and installed another one about 1/2" away, plus one on opposite side of the backing plate for balance. One of those planes came back the next year with no further problems. The second plane came back with a crack in the center mounting bolt hole and the whole backing plate was replaced.
These instances were a couple years before I bought the prop balancer.

I am helping a friend with a condition inspection on an RV8 with a BA Hartzell, and I found the same thing on the backplate. Two rivet heads popped off, both adjacent to one of the blades. No cracking noted anywhere. I replaced both rivets. The prop hasn?t been dynamically balanced.