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The last time I tried to rivet the spar, after 5 rivets in the center the spar started to bow out, so I stopped. This time, however, I started one rivet in the middle, and only set it about half-way (3 hits on the hammer). This was enough to hold the parts together without really clamping them tight. Then we moved to one end of the spar and did the same with another rivet. Next we riveted the other end of the spar half way. We kept setting rivets between the previously set rivets until the entire bottom spar was riveted "half-way". (Think of the torque sequence for an engine head).
After all rivets were set half way, we went back down the line and finished setting each rivet. This time, it took 5 more blows with the hammer to set them. The spar still came out a little bowed, but not bad, and only a small ammount for force will straighten it. Whew! I am so glad this method is working. A have all of the ribs made, so the spar was the only thing holding me back. Not any more!