After reading a number of posts on VAF website on how to rivet ribs to main spar, I decided on the inverted spar with bucking bar on the bench under the work. Set up blocks on either end of the spar, cleco'd on the first rib and inserted rivets so factory head was on the rib side. I used the straight set at 45 psi on the gun. The spar flexed enough so I could push the rivet down to hit the bucking bar underneath and I set the first rivet. System works very well as a solo operation, but I recruited Fran to come and help position the bucking bar, just for that extra security that it was staying centered. I ran the gun and moved the ribs out of the way, with Fran on the bucking bar and rivet gage. Got four done before dinner.
