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These pictures show both sides of the spar just before I finished the last of the rivets with the gun.

      These pictures show the “Sonex bow” that most everyone has reported after riveting the spars.  I would have been alarmed if I hadn’t known it would happen.  It is the result of differential expansion of material on the top and bottom as the rivet is formed.  The second picture is a smaller example that showed up in my test piece.

      In articles on riveting, I read of methods to minimize it but opted not to.  It takes very little force to make it straight.  I tested it by laying the spar with the bend up with a board  near the start of the thinning part of the spar cap.  I then laid a two pound sledge hammer at the tip and the spar was straight while the hammer was sitting there.