340 7-blade fan rivet missing

pishta said

replace all of them, balanced again!

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Good point.

My bet is the manufacturer did not balance the fans.

The rivets would all be the same weight within a very small tolerance.

The blades the same and punched out of steel.

The center, punched out of steel.

Now maybe they weighed the blades and used 7 blades that were all within a specific weight range.